Structural Study of TlH2PO4 and TlD2PO4 in the High Temperature Phase

Phonon dynamics simulation of TlH2PO4 crystal

“…In contrast to the other crystals of this family, TDP and its deuterated analogue TlD 2 PO 4 crystallize in the orthorhombic Pbcn [1] high-temperature structure. TDP undergoes two structural phase transitions with decreasing temperature at T l ¼ 357 and T N ¼ 230 K [1][2][3][4]. The hightemperature second-order Pbcn (Z ¼ 4) $ P2 1 /b (Z ¼ 4) phase transition occurs into the ferroelastic phase with U YZ ¼ U 4 spontaneous deformation.…”

“…H 1 protons are located on the shorter O 1 -H 1 ÁÁÁO 2 hydrogen bonds connecting the PO 4 groups in zigzag-type chains running along the c crystallographic axis. Longer O 3 -H 2 ÁÁÁO 4 hydrogen bonds containing the H 2 protons link the c-axis running chains of PO 4 groups into an (a, c) network of PO 4 groups and hydrogen bonds [1][2][3][4][5].…”

“…It is accepted [1][2][3][4] that both types of protons may, with equal probability, occupy the two possible positions on hydrogen bonds. The ferroelastic Pbcn $ P2 1 /b phase transition is accompanied by the ordering of the H 2 hydrogen atoms, whereas H 1 protons are still disordered on hydrogen bonds.…”

“…Careful structural research was carried out by different investigators in all temperature phases [1][2][3][4][5]. An exhaustive set of Brillouin scattering [6], elastic [7], resonance [8][9][10][11][12], dielectric [13][14][15][16] and spectroscopic [17,18] measurements have been performed over the last three decades.…”

“…To preserve the mirror symmetry, we supposed the H atoms to be deposited at the middle of hydrogen bonds. The experimental data concerning the crystal structure determined in two structural phases [1,4] were used in our simulation. Note that all values of model parameters were adjusted based on the lattice stability conditions.…”