Optical study of the successive phase transitions of n-alkyl ammonium dihydrogen phosphate crystals

Measurements of the birefringence and indicatrix rotation in a family of ammonium dihydrogen phosphate (ADP) related ferroelastic crystals ${\mathrm{C}}_{\mathit{n}}$${\mathrm{H}}_{2\mathit{n}+1\mathrm{}}$${\mathrm{NH}}_3$${\mathrm{H}}_2$${\mathrm{PO}}_4$ (abbreviated as ${\mathrm{C}}_{\mathit{n}}$ADP), with n=1–9 and 12, reveal successive phase transitions depending on the lengths of the alkyl ammonium chains. This, taken together with the results of investigations of the dielectric constant, thermal expansion, and elastic modulus in ${\mathrm{C}}_7$ADP, shows that intercalated layered structures are realized in this case with different packing of the chains for odd and even n between layers of ${\mathrm{PO}}_4$ tetrahedra linked two dimensionally by H bonds.