OPTICAL STUDY OF THE SUCCESSIVE PHASE-TRANSITIONS OF N-ALKYL AMMONIUMDIHYDROGEN PHOSPHATE CRYSTALS

Measurements of the birefringence and indicatrix rotation in a family of ammonium dihydrogen phosphate (ADP) related ferroelastic crystals C nH2n+1NH3H2PO4 (abbreviated as C(n)ADP), with n = 1-9 and 12, reveal s uccessive phase transitions depending on the lengths of the alkyl ammo nium chains. This, taken together with the results of investigations o f the dielectric constant, thermal expansion, and elastic modulus in C 7ADP, shows that intercalated layered structures are realized in this case with different packing of the chains for odd and even n between l ayers of PO4 tetrahedra linked two dimensionally by H bonds.