Na 2TiGeO 5: Crystal structure stability at low temperature and high pressure

Abstract

The temperature evolution of the lattice parameters measured from 295 to 125 K exhibits a small instability below T c≈278 K, indicating ferroelastic properties of Na 2TiGeO 5. The behavior is related to the specific crystal structure built of polyhedral layers with shared TiO 5 pyramids and GeO 4 tetrahedra, alternating with layers of Na + cations. Antiparallel alignment of the short apical titanyl bond in adjacent rows of the polyhedral layer gives rise to spontaneous strain, when a distortion of the TiO 5 groups occurs. Single-crystal structures determined at room temperature and 120 K suggest that {1 1 0} domains, developing below T c, entail a tetragonal-to-orthorhombic symmetry change. The mechanism is attributed to a shortening of the O–O distance between the polyhedral layers, and to minor shifts of the positions of the Ti atoms and the correlated oxygen atoms along the c-axis. The structure distortion, however, is too small to allow any unambiguous determination of the symmetry-breaking effects. The bulk modulus and its pressure derivative have been determined as B 0=89(2) GPa and B 0 ′ = 4.0 . A pressure-induced phase transformation takes place at P c≈12.5 GPa, presumably to an orthorhombic structure. The pressure effect on the transition temperature is given by Δ T c/Δ P≈1.76 K/GPa.