The temperature evolution of the lattice parameters measured from 295 to 125 K exhibits a sm
all instability below
Tc≈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. Antipar
allel alignment of the short apical titanyl bond in ad
jacent 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
Tc, 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 sm
all to
allow any unambiguous determination of the symmetry-breaking effects. The bulk modulus and its pressure derivative have been determined as
B0=89(2) GPa and
![View the MathML source View the MathML source](http://www.sciencedirect.com/cache/MiamiImageURL/B6TXR-4PPW79S-1-G/0?wchp=dGLbVlz-zSkWb)
. A pressure-induced phase transformation takes place at
Pc≈12.5 GPa, presumably to an orthorhombic structure. The pressure effect on the transition temperature is given by Δ
Tc/Δ
P≈1.76 K/GPa.