We have studied the normal-state transport properties, the resistivity ρ(T), and the Hall coefficient ${\mathit{R}}_{\mathit{H}}$(T) on two sets of samples of R${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{y}}$ compounds (R=Y,Sm) away from optimal doping. In the overdoped regime, ${\mathit{R}}_{\mathit{H}}$(T) obtained from Ca-doped ${\mathrm{SmBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{y}}$ shows a saturation at high temperature. While ${\mathit{R}}_{\mathit{H}}$(T) decreases with T at low temperature, it becomes temperature independent when the temperature exceeds a certain value ${\mathit{T}}_{\mathit{H}}^{\mathrm{*}}$. This characteristic ${\mathit{T}}_{\mathit{H}}^{\mathrm{*}}$ decreases with doping, and is only visible in our experimental temperature range when the samples are sufficiently doped. By assuming a universal functional form for all doping levels, the temperature dependence of the Hall coefficient can be scaled. Another characteristic temperature ${\mathit{T}}_{\mathrm{\rho }}^{\mathrm{*}}$ can be seen from the resistivity data in the underdoped regime, obtained from the oxygen-deficient ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{y}}$ samples. Above ${\mathit{T}}_{\mathrm{\rho }}^{\mathrm{*}}$, ρ(T) increases almost linearly with T, whereas below, it deviates from the linear behavior. We discuss the relationship between these two characteristic temperatures ${\mathit{T}}_{\mathit{H}}^{\mathrm{*}}$ and ${\mathit{T}}_{\mathrm{\rho }}^{\mathrm{*}}$ for different doping levels. For the underdoped samples, the resistivity is fitted well by the relation ρ(T)≊${\mathit{aR}}_{\mathit{H}}$(T)${\mathit{T}}^2$+b.

We find a normal temperature dependence for the Doppler broadening of the 230-eV resonance in the neutron cross section of ${}_{}{}^{65}{}_{}{}^{}\mathrm{Cu}$ atoms in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ between 10 and 300 K. No abrupt changes in the copper momentum spectrum corresponding to vibrations in the ab plane were observed in either the superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ or nonsuperconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.2}$ compounds. This result differs from ion-channeling experiments in which sharp changes in the channeling minimum yield and width of the rocking curve indicated irregular vibrational behavior near the superconducting critical temperature.

The LA[100] (a-axis) Brillouin backscattering phonon spectra have been measured as a function of temperature (20–370 K) in the mixed ferroelectric (FE)–antiferroelectric (AFE) system ${\mathrm{Rb}}_{1\mathrm{-}\mathit{x}}$(${\mathrm{ND}}_4$${)}_{\mathit{x}}$${\mathrm{D}}_2$${\mathrm{AsO}}_4$ (DRADA-x) for ammonium concentrations x=0, 0.10, and 0.28. The Brillouin frequency shift with decreasing temperature shows hardening (positive coupling) whose steepness decreases with higher ${\mathrm{ND}}_4$ content. For ${\mathrm{RbD}}_2$${\mathrm{AsO}}_4$ (DRDA), a Landau-Khalatnikov-like maximum (which persists in weaker form for x=0.10) was observed and the polarization relaxation time is estimated to be τ∼3.8×${10}^{\mathrm{-}12}$/(${\mathit{T}}_{\mathit{c}}$-T) s, where ${\mathit{T}}_{\mathit{c}}$ is the ferroelectric transition temperature. For both x=0.10 and 0.28, a broad damping peak anomaly which is stronger in x=0.28 was observed and can be connected with the dynamic order parameter fluctuations. Taking into account earlier NMR and dielectric results, we conclude that the local structure competition between FE ordering and AFE ordering is the origin of these broad damping anomalies.

A simple three-dimensional orthorhombic model of particles with displacive degrees of freedom interacting with the strain variables has been studied by the molecular-dynamics technique. The modulated degrees of freedom is coupled linearly to the shear and bilinearly to the bulk deformation component. The computer simulation has confirmed that the commensurate-incommensurate phase transition mechanism relies on nucleation and growth of stripples. Different configuration patterns of the initial commensurate phase revealed that the elastic domain walls between shear domains can be used to nucleate new discommensuration planes, and stripples are able to recover planar folding defects of the lattice, defects which are the result of heavy crystal deformation. It is shown that in the incommensurate phase the shear component forms a modulated field which follows the modulation wave with the same wave vector, but the bulk deformation component doubles the modulation wave vector.

The nuclear-resonance photon-scattering technique was used for studying the temperature dependence between 12 and 295 K of the anisotropy in the scattering cross section from the 7362-keV level in ${}_{}{}^{68}{}_{}{}^{}\mathrm{Zn}$ using a single crystal of metallic zinc. The anisotropy ratio of the scattering intensities with the beam parallel and perpendicular to the hexagonal planes of Zn was found to increase strongly between 295 and 12 K. This was interpreted in terms of the anisotropic binding of the Zn atoms and was used for deducing the mean-square momenta 〈${\mathit{p}}^2$${\mathrm{〉}}_{\mathrm{\perp }}$ and 〈${\mathit{p}}^2$${\mathrm{〉}}_{\mathrm{\parallel }}$ of the Zn atoms at 0 K perpendicular and parallel to the hexagonal planes, and the corresponding Debye temperatures ${\mathit{FTHETA}}_{\mathrm{\parallel }}$ and ${\mathit{FTHETA}}_{\mathrm{\perp }}$. When these values were combined with the known values of the zero-point vibrational amplitudes 〈${\mathit{x}}^2$${\mathrm{〉}}_{\mathrm{\perp }}$ and 〈${\mathit{x}}^2$${\mathrm{〉}}_{\mathrm{\parallel }}$ obtained using the Mössbauer effect, the theoretical lower limit of the uncertainty principle for a metallic solid was found to be reached almost accurately.

A general formalism is developed for describing nuclear-spin-lattice relaxation for quadrupolar perturbed nuclear magnetic resonance (NMR) in structurally incommensurately (IC) modulated crystals in terms of the elementary excitations of these systems. Our discussion deviates from previous ones as regards the identification of the amplitudon and phason induced relaxation rates 1/${\mathit{T}}_{1\mathit{A}}$ and 1/${\mathit{T}}_{1\mathrm{\phi }}$ in the plane wave limit as well as the temperature dependence of ${\mathit{T}}_{1\mathrm{\phi }}$ in the soliton limit. Measurements are reported for the spin-lattice relaxation of the ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$ satellite transitions in the normal, incommensurate, and commensurate phases of the prototype incommensurate system ${\mathrm{Rb}}_2$${\mathrm{ZnCl}}_4$. At ambient temperature the variation of the nuclear magnetic relaxation rate is measured over the incommensurately broadened distribution of resonance frequencies for two crystal orientations. The ${\mathit{T}}_1$ model developed is applied to these data, making use of the Fourier series of the static electric field gradient determined from the NMR satellite spectra in the IC phase. A strict agreement is found. The temperature dependence of ${\mathit{T}}_1$ is determined in that crystal orientation where a well-defined assignment of ${\mathit{T}}_{1\mathit{A}}$ and ${\mathit{T}}_{1\mathrm{\phi }}$ can be given. In the low-temperature part of the incommensurate phase, our data reflect the local softening of phase fluctuations in the discommensurations or the flattening out of the corresponding acoustic branch what is in accordance with our theoretical prediction and in contrast to previous results. The relevance of phason gaps previously derived from NMR data is discussed.

The stability of the γ’ phase of the nickel-base alloy Nimonic PE16 under irradiation with ${\mathrm{Ni}}^{+}$ ions of 300-keV energy is studied by means of transmission electron microscopy. The disordering of the γ’ phase could be followed as a function of fluence. The main finding is the observation of weak but measurable superlattice reflections between 0.1 and 1 dpa after irradiation at room temperature. The superlattice reflections disappear in two steps. Their intensities decrease considerably within a fluence of 0.1 dpa, while weak intensities are observed up to a fluence of 1 dpa. These reflections disappear completely after a fluence of 2 dpa. The results are discussed within a model which considers both disordering and dissolution of precipitates under cascade producing irradiation.

The crystal structure of the distorted-fcc high-pressure phase of the rare-earth elements is discussed in connection with the Landau theory of phase transition. From x-ray-diffraction spectra the irreducible representation ${\mathit{E}}_{\mathit{u}}^{\mathit{L}}$ is found to drive the (almost) continuous transition fcc→distorted fcc. Using just symmetry arguments four structures induced by ${\mathit{E}}_{\mathit{u}}^{\mathit{L}}$ are compatible with the x-ray-diffraction pattern; but only one (space group Cmmm, Z=8) can have a continuous transition to fcc.

Excitation of the transition ${}_{}{}^3{}_{}{}^{}$${\mathit{H}}_4$${\to }^1$${\mathit{D}}_2$ in a ${\mathrm{Pr}}^{3+}$-doped fluoroindate glass at ∼588 nm results in efficient blue emission at ∼480 nm which is ascribed to the ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_0$${\to }^3$${\mathit{H}}_4$. The upconversion process is due to an energy transfer involving a pair of ${\mathrm{Pr}}^{3+}$ ions. The dynamical behavior of the anti-Stokes emission is described using a rate-equation model which allows one to obtain the energy-transfer rates involving the pair states which contribute to the upconversion process.

An electron-paramagnetic-resonance (EPR) investigation at 90 K of rhodium impurities in thermally reduced KTP crystals is reported. The EPR spectra are assigned to two rhodium centers. It is proposed that the corresponding defects are ${\mathrm{Rh}}^{2+}$ ions substituting for titanium in the two Ti sites of the KTP structure. The spin-Hamiltonian parameters are analyzed and the possible mechanisms accounting for the distortion around the impurity are discussed. Reducing thermal treatments in vacuum of KTP crystals induce a strong increase of the observed EPR spectra, indicating that most rhodium impurities enter in KTP as ${\mathrm{Rh}}^{3+}$ ions.

The crystal structures of three calcium chalcogenides CaS, CaSe, and CaTe were investigated by energy-dispersive x-ray-diffraction techniques using a synchrotron source to a static compression up to 52 GPa. For CaS and CaSe, a first-order phase transformation from the NaCl phase to the CsCl phase was observed at 40 and 38 GPa, respectively. For CaTe, it first transforms from the NaCl phase to an intermediate state possibly with a mixture of NaCl and MnP phases at 25 GPa, and then to the CsCl phase above 33 GPa. The phase transitions were reversible with small hysteresis. The equations of state and bulk moduli were obtained for all three compounds. The systematic behavior among the calcium chalcogenides under compression and the unusual transition sequence of CaTe are discussed.

The crystal structure of Ba has been studied under pressures of up to 90 GPa at room temperature using powder x-ray-diffraction techniques. Phases I and II have been perfectly explained by the bcc and hcp structures, respectively. However, we obtained no evidence for the existence of phase III, which has been previously reported between 8 and 12.6 GPa at room temperature. The powder pattern of phase IV was very spotty and its structure has remained unsolved. The structure of phase V above 45 GPa has been confirmed to be hcp. Ba therefore has two hcp phases under pressure: phase II (5.5–12.6 GPa) and phase V (above 45 GPa). The c/a ratio of phase II decreases dramatically with pressure, while that of phase V is pressure-independent with a value of 1.575. The mechanisms to stabilize the two hcp structures are expected to be very different.

A number of isotope effects can be seen in some hydrogen-bonded crystals as deuterium atoms are substituted for hydrogen atoms. In ${\mathrm{KH}}_2$${\mathrm{PO}}_4$-type crystals in particular, a rise in the ferroelectric transition temperature, an increase in spontaneous polarization, and an expansion of the hydrogen-bonded distance can be seen. The mechanism behind these isotope effects is explained theoretically by introducing a relation form between the hydrogen-bonded distance and distortions of ${\mathrm{PO}}_4$ ions without reference to proton tunneling.

The probabilities of creation and decay of ${\mathrm{La}}^{4+}$ 4${\mathit{d}}^9$ and ${\mathrm{La}}^{3+}$ 4${\mathit{d}}^9$4${\mathit{f}}^1$ configurations are calculated using a multiconfiguration, Dirac-Fock code. Both x-ray- and Auger-emission spectra are simulated and compared with experimental electron-induced spectra. This comparison leads to an original interpretation of the spectra. We show that the decay rate of the excited 4${\mathit{d}}^9$4${\mathit{f}}^1$ state to discrete states is larger than that of the 4${\mathit{d}}^9$ ionized state. In particular, for the ${\mathrm{La}}^{3+}$ 4${\mathit{d}}^9$4${\mathit{f}}^1$ ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}_1$ level that had been attributed to a ‘‘giant resonance,’’ we find a decay rate to discrete states a hundred times greater than that of 4${\mathit{d}}^9$. A comparison of the 4${\mathit{d}}^9$4${\mathit{f}}^1$ and 4${\mathit{d}}^9$ confiugration energies as well as values of the cross section of the ground state to each of these two high configurations is made for the three cases of La, ${\mathrm{La}}^{2+}$, and ${\mathrm{La}}^{3+}$.

Single crystals of ${\mathrm{Cs}}_3$${\mathrm{Lu}}_2$${\mathrm{Br}}_9$:1%${\mathrm{Er}}^{3+}$ were grown using the Bridgman technique. The low phonon energy of ≤190 ${\mathrm{cm}}^{\mathrm{-}1}$ strongly suppresses multiphonon relaxation in this host. For a series of excited states radiative relaxation is found to be dominant. As a consequence, ${\mathrm{Cs}}_3$${\mathrm{Lu}}_2$${\mathrm{Br}}_9$:1%${\mathrm{Er}}^{3+}$ shows strong visible upconversion luminescence from ${}_{}{}^2{}_{}{}^{}$${\mathit{H}}_{9/2}$ and ${}_{}{}^4{}_{}{}^{}$${\mathit{F}}_{7/2}$ when excited into the ${}_{}{}^4{}_{}{}^{}$${\mathit{I}}_{9/2}$ and ${}_{}{}^4{}_{}{}^{}$${\mathit{I}}_{11/2}$ multiplets in the near infrared, respectively. The upconversion dynamics for both excitations was studied in detail using time-resolved spectroscopy. Both excited-state absorption (ESA) and energy-transfer upconversion (ETU) were found to be active and could be identified on the basis of the upconversion transients. For the population of the ${}_{}{}^4{}_{}{}^{}$${\mathit{F}}_{7/2}$ and ${}_{}{}^4{}_{}{}^{}$${\mathit{S}}_{3/2}$ states both ESA and ETU can be observed simultaneously. The relative efficiency of ESA and ETU is determined by the energetic resonance of the respective processes. Whereas nonresonant ESA during the 10 ns laser pulse is strongly suppressed for energy mismatches >12 ${\mathrm{cm}}^{\mathrm{-}1}$, ETU is still efficient. An analysis of the upconversion transients shows that energy migration is dominant even at 1% ${\mathrm{Er}}^{3+}$ dotation. The excitations have excitonic character leading to a characteristic prolongation of the decays in the upconversion transients.

X-ray diffraction of black phosphorus and phosphorus-arsenic alloys with composition ${\mathrm{P}}_{1\mathrm{-}\mathit{x}}$${\mathrm{As}}_{\mathit{x}}$ (x=0.05,0.1) has been carried out with synchrotron radiation at low temperatures and high pressures. The first transition from the orthorhombic to the rhombohedral state started at around 7.5 GPa at 21 K. The transition pressure at 21 K is about 3 GPa higher than that at room temperature. The orthorhombic and rhombohedral phases coexisted over the pressure range of 7.5–10.5 GPa at 21 K. The second transition to a simple-cubic phase appeared at around 12 GPa. The rhombohedral and the simple-cubic phases coexisted over the pressure range of 12–15.5 GPa at 21 K. This region was much wider than that observed at room temperature. Similar behavior was found for ${\mathrm{P}}_{0.95}$${\mathrm{As}}_{0.05}$ and ${\mathrm{P}}_{0.9}$${\mathrm{As}}_{0.1}$ at low temperatures and high pressures. Superconductivity of black phosphorus and the alloys has been investigated at low temperatures and high pressures. Interestingly, superconductivity is observed when pressure is increased at liquid-helium temperature. The superconducting transition temperature (${\mathit{T}}_{\mathit{c}}$) of these materials rapidly increased to about 10 K at around 25 GPa. The single phase of the simple cubic form was observed at around 16 GPa for black P and 22 GPa for ${\mathrm{P}}_{0.9}$${\mathrm{As}}_{0.1}$ at 21 K. A bulk thermal expansion coefficient of the simple-cubic phase was approximately 13×${10}^{\mathrm{-}6}$ (deg${)}^{\mathrm{-}1}$. This value is considerably smaller than that of ordinary metals. The small thermal expansion is due to the instability of the simple-cubic structure at low temperatures. The high ${\mathit{T}}_{\mathit{c}}$ of the simple-cubic structure may arise from the lattice instability at very low temperatures.

The molecular-dynamics studies reported in this work concern the behavior of a system, with a pair-potential model appropriate for CuI, as a function of its charge ‖Z‖. These studies are carried out at two temperatures. The lower at T=625 K, corresponds to its γ phase; the higher, at T=700 K, to its α phase. The structure of the system in the γ phase at ‖Z‖=0.6 is zinc-blende. When the charge is lowered to ‖Z‖=0.3 the system is found in a rocksalt structure. At ‖Z‖=0.2 the structure is still rocksalt, but the cations now diffuse through hopping. At ‖Z‖=0 it is found in the molten state. In the α phase, at ‖Z‖=0.6, the system is superionic with the anions sitting on a fcc sublattice while the cations exhibit a liquidlike structure. For ‖Z‖=0.2, the system is found in a rocksalt structure, and is no longer superionic. When the charge is completely removed the system is again found in the molten state. The lowering of the charge in this model almost-ionic solid appears to exhibit effects akin to those found by increasing its pressure.

A metal-semiconductor transition at ${\mathit{T}}_{\mathit{c}}$=170.4 K in a two-dimensional organic metal, (BEDT-TTF${)}_3$(${\mathrm{ClO}}_4$${)}_2$, was investigated by means of ESR, resistivities, specific heats, and x-ray diffractions. The specific heats show a λ-type anomaly at ${\mathit{T}}_{\mathit{c}}$ with a second-order nature and a large entropy change range of 3 J/K mol. In addition to the H/D isotope effects on ${\mathit{T}}_{\mathit{c}}$, x-ray diffraction suggests the development of an intermolecular interaction network between the ethylene groups of BEDT-TTF molecules and ${\mathrm{ClO}}_4^{\mathrm{-}}$ anions below ${\mathit{T}}_{\mathit{c}}$. These experimental findings demonstrate that the three-dimensional structural rearrangement, which is closely correlated to the development of the intermolecular interaction network, plays an important role in the metal-semiconductor transition.