This paper is a contribution to the joint Physical Review Applied and Physical Review Materials collection titled Two-Dimensional Materials and Devices.

Metal and magnetic overlayers alter the surface of the topological insulator (TI) bismuth selenide $\left(\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3\right)$ through proximity effects but also by changing the composition and chemical structure of the $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ subsurface. The interface between $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ and Mn metal or manganese selenide was explored using x-ray photoelectron spectroscopy revealing chemical and electronic changes at the interface. Depositing Mn metal on $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ without an external source of Se shows unexpected bonding within the Mn layer due to Mn-Se bonding as Se diffuses out of the $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ layer into the growing Mn film. The Se outdiffusion is further evidenced by changes in Bi core levels within the $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ layers indicating primarily Bi-Bi bonding over Bi-Se bonding. No outdiffusion of Se occurred when excess Se is supplied with Mn, indicating the importance of supplying enough chalcogen atoms with deposited metals. However, $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ core-level photoelectrons exhibited a rigid chemical shift toward higher binding energy after depositing a monolayer of $\mathrm{MnS}{\mathrm{e}}_{2-x}$, indicating a dipole within the overlayer. Stoichiometry calculations indicated that the monolayer forms MnSe preferentially over the transition metal dichalcogenide (TMD) phase $\mathrm{MnS}{\mathrm{e}}_2$, providing a consistent picture of the dipole formation in which a plane of Se anions sits above Mn cations. This study shows that chemical diffusion and dipole formation are important for $\mathrm{Mn}\text{−}\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ and $\mathrm{MnS}{\mathrm{e}}_{2-x}\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ and should be considered carefully for TMD/TI interfaces more generally.

The kinetics and mechanisms of phase transitions that occur during lithiation of amorphous silicon thin films were extensively studied using electrochemical techniques and transmission electron microscopy. Measurements of current made while changing applied potentials at a constant rate (cyclic voltammograms) showed peaks that correspond to the formation of two amorphous Li-Si alloys with different stoichiometries. Peaks associated with these phase transitions were also observed in current-time measurements made in potentiostatic experiments. The times at which currents reached maxima were found to be independent of film thickness, suggesting that the phase transitions occur throughout the volume of the films. Analysis of current-time measurements using the Johnson-Mehl-Avrami-Kolmogorov method indicate that the transitions occur through a first-order nucleation and growth process. This conclusion is supported by cross-section transmission electron microscopy in which high-energy electron-beam-induced sputtering of lithium led to contrast between the parent and product phases. In contrast with other studies, the polyamorphic amorphous-to-amorphous first-order phase transitions observed in this study are driven by changes in composition rather than changes in pressure or temperature. The potentiostatic techniques employed in this study can be used to characterize the nature and kinetics of phase transitions that occur in other electrode materials for lithium-ion batteries.

Reversible topotactic transitions between oxygen-vacancy-ordered structures in transition-metal oxides provide a promising strategy for active manipulation of material properties. While transformations between various oxygen-deficient phases have been attained in bulk $\mathrm{AB}{\mathrm{O}}_{3-\delta }$ perovskites, substrate clamping restricts the formation of distinct ordering patterns in epitaxial films. Using in situ scanning transmission electron microscopy (STEM), we image a thermally driven reversible transition in $\mathrm{L}{\mathrm{a}}_{0.5}\mathrm{S}{\mathrm{r}}_{0.5}\mathrm{Co}{\mathrm{O}}_{3-\delta }$ films on $\mathrm{SrTi}{\mathrm{O}}_3$ from a multidomain brownmillerite (BM) structure to a uniform new phase wherein oxygen vacancies order in every third $\mathrm{Co}{\mathrm{O}}_x$ plane. Because temperature cycling is performed over a limited temperature range $(25-385{}^{\circ }\mathrm{C})$, the oxygen deficiency parameter $\delta$ does not vary measurably. Under constant $\delta$, the topotactic transition proceeds via local reordering of oxygen vacancies driven by thermal strain. Atomic-resolution imaging reveals a two-step process whereby alternating vertically and horizontally oriented BM domains first scale in size to accommodate the strain induced by different thermal expansions of $\mathrm{L}{\mathrm{a}}_{0.5}\mathrm{S}{\mathrm{r}}_{0.5}\mathrm{Co}{\mathrm{O}}_{3-\delta }$ and $\mathrm{SrTi}{\mathrm{O}}_3$, before the new phase nucleates and quickly grows above $360{}^{\circ }\mathrm{C}$. Upon cooling, the film transforms back to the mixed BM phase. As the structural transition is fully reversible and $\delta$ does not change upon temperature cycling, we rule out electron-beam irradiation during STEM as the driving mechanism. Instead, our findings demonstrate that thermal strain can solely drive topotactic phase transitions in perovskite oxide films, presenting opportunities for switchable ionic devices.

Three-dimensional topological insulators (3D TIs) possess a specific topological order of electronic bands, resulting in gapless surface states via bulk-edge correspondence. Exotic phenomena have been realized in ferromagnetic TIs, such as the quantum anomalous Hall (QAH) effect with a chiral-edge conduction and a quantized value of the Hall resistance $R_{yx}$. Here, we report on the emergence of distinct topological phases in paramagnetic Fe-doped ${\left(\mathrm{Bi},\mathrm{Sb}\right)}_2\mathrm{S}{\mathrm{e}}_3$ heterostructures with varying structure architecture, doping, and magnetic and electric fields. Starting from a 3D TI, a two-dimensional insulator appears at layer thicknesses below a critical value, which turns into an Anderson insulator for Fe concentrations sufficiently large to produce localization by magnetic disorder. With applying a magnetic field, a topological transition from the Anderson insulator to the QAH state occurs, which is driven by the formation of an exchange gap owing to a giant Zeeman splitting and reduced magnetic disorder. A topological phase diagram of ${\left(\mathrm{Bi},\mathrm{Sb}\right)}_2\mathrm{S}{\mathrm{e}}_3$ allows exploration of intricate interplay of topological protection, magnetic disorder, and exchange splitting.

Two-dimensional electron gases (2DEGs) can form at the surface of oxides and semiconductors or in carefully designed quantum wells and interfaces. Depending on the shape of the confining potential, 2DEGs may experience a finite electric field, which gives rise to relativistic effects such as the Rashba spin-orbit coupling. Although the amplitude of this electric field can be modulated by an external gate voltage, which in turn tunes the 2DEG carrier density, sheet resistance and other related properties, this modulation is volatile. Here, we report the design of a “ferroelectric” 2DEG whose transport properties can be electrostatically switched in a nonvolatile way. We generate a 2DEG by depositing a thin Al layer onto a $\mathrm{SrTi}{\mathrm{O}}_3$ single crystal in which 1% of Sr is substituted by Ca to make it ferroelectric. Signatures of the ferroelectric phase transition at 25 K are visible in the Raman response and in the temperature dependences of the carrier density and sheet resistance that shows a hysteretic dependence on electric field as a consequence of ferroelectricity. We suggest that this behavior may be extended to other oxide 2DEGs, leading to novel types of ferromagnet-free spintronic architectures.

Materials with simple crystal structure usually exhibit substantial lattice thermal conductivity $\left({\kappa }_{\mathrm{latt}}\right)$ due to a scarcity of strong phonon-phonon scattering channels. Yet here we find simple CsCl-structure TlBr to have an extremely low room temperature ${\kappa }_{\mathrm{latt}}\sim 0.5\mathrm{W}/\text{m-K}$, while at low temperature we find a high ${\kappa }_{\mathrm{latt}}$ of $155\mathrm{W}/\text{m-K}$. This extreme range of conductivity behavior results from lattice-instability-related anharmonicity, effective only at temperatures above $\sim 30\mathrm{K}$. Manipulating these instabilities (e.g., pressure, strain) may yield a pathway to ultralow thermal conductivity via intrinsic phonon resistance in similar materials.

At its boundaries and vortices, a topological superconductor features Majorana fermions, which are potentially applicable for topological quantum computations. The scarcity of the known experimentally verified physical systems with topological superconductivity, time-reversal invariant ones in particular, is giving rise to a strong demand for identifying new candidate materials. In this research, we study a heterostructure consisting of a transition-metal-oxide two-dimensional electron gas (2DEG) sandwiched by insulators near the paraelectric/ferroelectric (PE/FE) phase transition. Its relevant characteristics are the combination of the transition-metal spin-orbit coupling and the soft odd-parity phonons arising from the ferroelectric fluctuation; it gives rise to the fluctuating Rashba effect, which can mediate the pairing interaction for time-reversal invariant topological superconductivity. As the PE/FE phase transition can be driven by applying strain on the heterostructure, this system provides a tunable electron-phonon coupling. Through the first-principle calculations on the (001) $\left[{\mathrm{BaOsO}}_3\right]{\left[{\mathrm{BaTiO}}_3\right]}_4$, we find such electron-phonon coupling to be strong over a wide range of applied tensile biaxial strain in the monolayer ${\mathrm{BaOsO}}_3$ sandwiched between the (001) ${\mathrm{BaTiO}}_3$, hence qualifying it as a good candidate material. Furthermore, the stability of topological superconductivity in this material is enhanced by its orbital physics that gives rise to the anisotropic dispersion.

This paper is a contribution to the joint Physical Review Applied and Physical Review Materials collection titled Two-Dimensional Materials and Devices.

We perform coherent nonlinear spectroscopy of excitons in single-layers of ${\mathrm{MoSe}}_2$ and ${\mathrm{WSe}}_2$ encapsulated between thin films of hexagonal boron nitride. Employing four-wave-mixing microscopy we identify virtually disorder-free areas, generating an exciton optical response at the homogeneous limit. Focussing on such areas, we measure exciton homogeneous broadening as a function of environmental factors, namely temperature and exciton density. Exploiting four-wave-mixing imaging, we find that at such locations, nonlinear absorption of the exciton excited states and their coherent couplings can be observed. Using the ${\mathrm{WSe}}_2$ heterostructure, we infer coherence and density dynamics of the exciton $2S$ state. Owing to its increased radiative lifetime, at low temperatures, the dephasing of the $2S$ state is longer than of the $1S$ transition. While scanning various heterostructures across tens of micrometers, we conclude that the disorder, principally induced by strain variations, remain to be present creating spatially varying inhomogeneous broadening.

Phosphorene is a promising two-dimensional material for electronics, energy, and catalysis applications, but its instability in ambient conditions is a major obstacle in its commercialization. The degradation begins with dissociation of ${\mathrm{O}}_2$ onto the surface of phosphorene which is spontaneous in its singlet state. Focusing on singlet ${\mathrm{O}}_2$ splitting, compressive strain is proposed as a method of inducing an activation barrier to this process. Applying compressive strain introduced a barrier as large as 0.45 eV causing a decrease in the rate constant by seven orders of magnitude. Introducing entropic effects via metadynamics biased ab initio molecular dynamics allowed for the generation of free-energy landscapes. Through this method, the largest barrier was observed to be 0.33 ± 0.04 eV causing a decrease in the kinetic rate constant by five orders of magnitude. These results highlight compression as a method of inhibiting singlet oxidation of phosphorene and pave the way towards further experimental investigation.

It has been long hypothesized that the structure of a material bombarded by energetic particles might approach a certain asymptotic steady state in the limit of high exposure to irradiation. There is still no definitive verdict regarding the validity of this hypothesis or the conditions where it applies. To clarify this, we explore a highly simplified model for microstructural evolution that retains full atomic detail of the underlying crystal structure and involves random events of generation and relaxation of defects. We explore the dynamics of evolution of the model in the limit $T=0$, where the defect and dislocation microstructure is driven purely by the spatially fluctuating stress field accumulating as a result of stochastic generation of point defects. Using body-centred cubic iron and tungsten as examples, we show that their microstructure exhibits a structural transition and then approaches a limiting asymptotic state at doses of order $O\left(0.1\right)$ and $O\left(1\right)$ canonical defects per atom, respectively, and analyze the microscopic and macroscopic parameters characterizing both the transition and the asymptotic microstructural state.

When exposed to air, metallic yttrium dihydride ${\mathrm{YH}}_2$ films turn into insulating and transparent yttrium oxyhydride (YHO). The incorporation of oxygen causes the lattice expansion of ${\mathrm{YH}}_2$ and the emergence of photochromic properties, i.e., YHO darkens reversibly when illuminated with light of adequate energy and intensity. However, the adequate bleaching of the photodarkened samples once the illumination has stopped is much faster in air than in inert atmosphere. According to this experimental evidence, the photochromic mechanism has to be related to an oxygen diffusion and exchange process. Since this process is accompanied by a lattice expansion/contraction, it can be said that YHO “breathes” when subjected to illumination/darkness cycling. Another interesting side effect of the breathing is the unexpected enhancement of the hydrophobicity of the YHO samples under illumination. A theoretical model able to explain the breathing in YHO is presented, together with the discussion of other alternative explanations.

Perovskite oxides used in heterogeneous catalysis and electrocatalysis are tuned by substitutional doping. Sr-doped $\mathrm{LaMn}{\mathrm{O}}_3$ is a popular choice of electrode material for electrochemical energy conversion. At elevated temperatures, relevant to solid oxide fuel or electrolysis cells, Sr enriches at the surface, which leads to fast degradation of the oxygen exchange activity at the surface. In this work, we investigate the effect of oxygen partial pressure $p\left(O_2\right)$ and temperature on Sr segregation at the $\mathrm{L}{\mathrm{a}}_{0.75}\mathrm{S}{\mathrm{r}}_{0.25}\mathrm{Mn}{\mathrm{O}}_3\left(001\right)$ (LSM25) surface by ab initio thermodynamics calculations, taking into account different terminations and point defects ${\mathrm{Sr}}_{\mathrm{La}}^{‴}$, ${\mathrm{V}}_{\mathrm{O}}^{··}$, ${\mathrm{V}}_{\mathrm{Mn}}^{‴}$, ${\mathrm{V}}_{\mathrm{La}}^{‴}$, ${\mathrm{O}}_{\mathrm{ad}}^{}$, as well as associated defects of the type $[{\mathrm{V}}_{\mathrm{O}}^{··}-{\mathrm{V}}_{\mathrm{cation}}^{‴}]$. This model of the LSM25(001) surface addresses the connection between point defects and surface stability and makes quantitative predictions about the surface termination and Sr enrichment. Our results indicate that the $\mathrm{Mn}{\mathrm{O}}_2$ termination is stable under oxidizing conditions, while a partially covering SrO termination on $\mathrm{Mn}{\mathrm{O}}_2$ is stable for reaction conditions between 1000 and 1400 K and effective oxygen partial pressures between ${10}^{-11}\mathrm{bar}$ and 0.1 bar relevant to solid oxide fuel cell (SOFC) cathodes. Under more reducing conditions, we find that Sr is enriched at the surface regardless of surface termination. This trend is a direct consequence of the increased formation of positively charged oxygen vacancies at the surface, and this conclusion holds regardless of which surface termination is assumed. The partial SrO termination is exceptionally stable under SOFC cathode conditions because it minimizes the surface dipole density, while all the defect-free terminations of LSM25(001) are unstable due to surface polarity. This newly proposed termination leaves both Sr, as well as Mn and O sites of the $\mathrm{Mn}{\mathrm{O}}_2$ layer exposed at the surface and available for interaction with the gas phase. We propose that the LSM25(001) surface may consist of coexisting (La,Sr)O and $\mathrm{Mn}{\mathrm{O}}_2$ domains under SOFC cathode conditions, and we propose to include such a micropatterned surface in the future discussion of the active site in the oxygen reduction reaction.

Ferrimagnetic iron garnets are promising materials for spintronics applications, characterized by ultralow damping and zero current shunting. It has recently been found that few nm-thick garnet films interfaced with a heavy metal can also exhibit sizable interfacial spin-orbit interactions, leading to the emergence, and efficient electrical control, of one-dimensional chiral domain walls. Two-dimensional bubbles, by contrast, have so far only been confirmed in micrometer-thick films. Here, we show by high resolution scanning transmission x-ray microscopy and photoemission electron microscopy that submicrometer bubbles can be nucleated and stabilized in $\sim 25$-nm-thick thulium iron garnet films via short heat pulses generated by electric current in an adjacent Pt strip, or by ultrafast laser illumination. We also find that quasistatic processes do not lead to the formation of a bubble state, suggesting that the thermodynamic path to reaching that state requires transient dynamics. X-ray imaging reveals that the bubbles have Bloch-type walls with random chirality and topology, indicating negligible chiral interactions at the garnet film thickness studied here. The robustness of thermal nucleation and the feasibility demonstrated here to image garnet-based devices by x-rays both in transmission geometry and with sensitivity to the domain wall chirality are critical steps to enabling the study of small spin textures and dynamics in perpendicularly magnetized thin-film garnets.

Recently, remarkable experimental progress has been made toward understanding the metastable phases in Mg-Sn alloys, a promising candidate for rare-earth free Mg alloys. However, the detailed structures, stability, and formation mechanisms of these metastable phases at early stages of precipitation remain unclear. Here, we report a first-principles study of the ground-state and finite-temperature phase stability of hexagonal closest packed (HCP)- and face-centered cubic (FCC)-based phases in Mg-Sn alloys. The ground-state phases are searched by the cluster expansion approach coupled with energy input from first-principles calculations. The Monte Carlo simulations with Hamiltonian from the cluster expansion are performed to calculate the configurational free energies and the corresponding metastable phase diagrams are constructed. Our calculations reveal rich varieties of orderings on HCP and FCC lattices, metastable ${\mathrm{Mg}}_3\mathrm{Sn}$ phases with $\mathrm{L}{1}_2$ and $\mathrm{D}{0}_{19}$ structures, and possible precursors for Guinier-Preston (G.P.) zones. Further analysis of the vibrational free energy indicates that $\mathrm{L}{1}_2$ ${\mathrm{Mg}}_3\mathrm{Sn}$ is stabilized by vibrational entropy at high temperatures. A new precipitation sequence for Mg-Sn alloys is suggested, i.e., supersaturated solid solution → G.P. zone → $\mathrm{D}{0}_{19}$ ${\mathrm{Mg}}_3\mathrm{Sn}\to \mathrm{L}{1}_2$ ${\mathrm{Mg}}_3\mathrm{Sn}\to \beta {\mathrm{Mg}}_2\mathrm{Sn}$.

Materials with increased functionality are often based on crystalline structures with significant local disorder. Typical examples are ferroic oxides exhibiting large spontaneous polarization that can be rotated by an applied electric field, finding use in many important applications. Despite years of investigation, the exact structural origin of the increased piezoelectric response of oxide ferroics is still unclear. Frequently evoked models attribute it to emerging polar nanoregions inside a nonpolar matrix, the existence of a morphotropic boundary separating polar phases with different crystallographic symmetry, low-symmetry bridging phases facilitating polarization rotation, and displacive and order-disorder structural phase transitions. Here we use both conventional and resonant high-energy x-ray diffraction coupled to atomic pair distribution function analysis and three-dimensional computer simulations to examine the relationship between the local structure and piezoelectric properties of exemplary sodium-potassium niobate ferroics. We show that their increased piezoelectric response is primarily due to a geometrical frustration in the underlying perovskite lattice induced by local fluctuations in the tilt pattern of the constituent niobium-oxygen octahedra, and not to a crystal-crystal phase transition or distinct nanodomains. The fluctuations peak when the sodium to potassium ratio approaches 1, leading to a softening of the perovskite lattice and easing of polarization rotation under an electric field. Based on the experimental data and model calculations involving Goldschmidt's tolerance factor for the stability of perovskites, we also show that the fluctuations are driven by the mismatch between the radii of sodium and potassium atoms, and the increased piezoelectric response of sodium-potassium niobates indeed scales with the variance in the distribution of these radii about the average value. Thus, we settle important aspects of the debate over the structure-piezoelectric property relationship for oxide ferroics, thereby providing a different perspective on the ongoing effort to improve their useful properties.

This work provides a direct route to measure the degree of hybridization of $f$ states in rare earths. The interference between electric dipole and octupole transitions is measured at the $L_1$ edge of Gd in ${\mathrm{Gd}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ using x-ray natural linear dichroism (XNLD) and high energy resolution fluorescence detection. The Gd $4f\text{−}6p$ admixture is quantified through the integral of the dipole-octupole XNLD using a sum rule easily applicable to experimental data. The mixing of the Gd valence states with the O ligand orbitals, calculated from first principles, reveals that despite their localized character, the Gd $4f$ orbitals mix with the O $2p$ and $2s$ orbitals with an antibonding and bonding character, respectively.

Silicene, a silicon analog of graphene, has the potential to become a candidate next-generation material by virtue of its novel physical and chemical properties. Although a rich variety of rotationally nonequivalent silicene structures have been observed with silicene grown on Ag(111) surfaces, the $T$ phase, which has been considered a precursor phase, has a less-ordered structure, and thus, it is difficult to clarify its atomic structure. In this paper, we report the atomic structures of the $T$ phase observed by high-resolution atomic force microscopy (AFM). While scanning tunneling microscopy images of the $T$ phase show characteristic round dots, AFM reveals that the $T$ phase has a continuous Si honeycomb arrangement, thus forming silicene. We identify two types of $T$ phases with different silicene rotation angles with respect to the Ag substrate: the ($\sqrt{13}\times \sqrt{13}$) type-I $T$ phase and the tiling-pattern $T$ phase. The former has a unit cell corresponding to the conventionally proposed ($\sqrt{13}\times \sqrt{13}$) type I with less periodicity of Si buckling, while the latter is identified by the tessellation with four different rhombuses. We also investigate the $T$ phase by site-specific force spectroscopy and find that some Si atoms at the $T$ phase have slightly different chemical reactivities.

We report on extensive investigations of magnetism at the $n$-type $\mathrm{LaAl}{\mathrm{O}}_3\text{/}\mathrm{SrTi}{\mathrm{O}}_3$ interface performed by utilizing a spectrum of local and integrative analytical techniques: Scanning superconducting quantum interference device microscopy, polar Kerr magnetometry, ferromagnetic resonance, and magnetic torque magnetometry. The samples originated from the same wafers. For nominally fully oxidized samples, we find that the mere presence of the conducting interface does not induce magnetism to values exceeding already present magnetic signals originating from the substrates, irrespective of the measurement technique. With the controlled introduction of oxygen vacancies, however, the different analytical techniques with their inherently different sensitivities and potential interactions with possible magnetic moments in the samples yield different results concerning the existence of magnetism. These unexpected differences obtained with the various measurement techniques are a possible source of the disagreement in the literature about the existence of ferromagnetism at $\mathrm{LaAl}{\mathrm{O}}_3\text{/}\mathrm{SrTi}{\mathrm{O}}_3$ interfaces.

Realizing quantum materials in few atomic layer morphologies is a key to both observing and controlling a wide variety of exotic quantum phenomena. This includes topological electronic materials, where the tunability and dimensionality of few layer materials have enabled the detection of $Z_2$, Chern, and Majorana phases. Here we report the development of a platform for thin film correlated, topological states in the magnetic rare-earth monopnictide ($RX$) system GdBi synthesized by molecular beam epitaxy. This material is known from bulk single crystal studies to be semimetallic antiferromagnets with Neel temperature $T_{\mathrm{N}}=28\mathrm{K}$ and is the magnetic analog of the non-$f$-electron containing system LaBi proposed to have topological surface states. Our transport and magnetization studies of thin films grown epitaxially on ${\mathrm{BaF}}_2$ reveal that semimetallicity is lifted below approximately eight crystallographic unit cells while magnetic order is maintained down to our minimum thickness of five crystallographic unit cells. First-principles calculations show that the nontrivial topology is preserved down to the monolayer limit, where quantum confinement and the lattice symmetry give rise to a  $C=2$ Chern insulator phase. We further demonstrate the stabilization of these films against atmospheric degradation using a combination of air-free buffer and capping procedures. These results together identify thin-film $RX$ materials as potential platforms for engineering topological electronic bands in correlated magnetic materials.

The rapid degradation of the functional properties of many Ti-based alloys is due to the precipitation of the $\omega$ phase. In the conventional high-temperature shape memory alloy Ti-Ta, the formation of this phase compromises completely the shape memory effect, and high ($>100{}^{\circ }\text{C}$) transformation temperatures cannot be maintained during cycling. A solution to this problem is the addition of other elements to form Ti-Ta-$X$ alloys, which often modifies the transformation temperatures; due to the largely unexplored space of possible compositions, very few elements are known to stabilize the shape memory effect without decreasing the transformation temperatures below $100{}^{\circ }\text{C}$. In this study, we use transparent descriptors derived from first-principles calculations to search for new ternary Ti-Ta-$X$ alloys that combine stability and high temperatures. We suggest four alloys with these properties, namely Ti-Ta-Sb, Ti-Ta-Bi, Ti-Ta-In, and Ti-Ta-Sc. Our predictions for the most promising of these alloys, Ti-Ta-Sc, are subsequently fully validated by experimental investigations, the alloy Ti-Ta-Sc showing no traces of $\omega$ phase after cycling. Our computational strategy is transferable to other materials and may contribute to suppress $\omega$ phase formation in a large class of alloys.