Neutron diffraction study of (ND4)2SeCl6, (ND4)2PtCl6, and (ND4)2PtBr6 crystals: The origin of the strong deuterium substitution effect on the phase transitions

Neutron powder diffraction experiments of ${\mathrm{(ND}}_{\mathrm{4}}{\mathrm{)}}_{\mathrm{2}}{\mathrm{SeCl}}_{\mathrm{6}},$ ${\mathrm{(ND}}_{\mathrm{4}}{\mathrm{)}}_{\mathrm{2}}{\mathrm{PtCl}}_{\mathrm{6}},$ and ${\mathrm{(ND}}_{\mathrm{4}}{\mathrm{)}}_{\mathrm{2}}{\mathrm{PtBr}}_{\mathrm{6}}$ were performed to investigate the mechanism of the strong deuterium substitution effect on the phase transitions of the ${\mathrm{(NH}}_{\mathrm{4}}{\mathrm{)}}_{\mathrm{2}}{\mathrm{MX}}_{\mathrm{6}}$ family. The isotope effect is strong in the first and second compounds and weak in the third. The data were collected in the $d$ -spacing range 0.5–4.3 Å by using a time-of-flight powder diffractometer VEGA installed at the pulsed cold neutron source in KEK. The intensity data of high-temperature phases (HTP) were measured at the temperatures corresponding to ${\text{1.3T}}_{\mathrm{c}}$ ($T_{\mathrm{c}}:$ transition temperature), and the low-temperature phases (LTP) at 4 K. The HTPs of the three compounds have an antifluorite type cubic structure ($\text{a≈9.8\hspace{0.17em}Å}$ and $\text{Z=4}$ ) as previously reported while the LTPs of ${\mathrm{(ND}}_{\mathrm{4}}{\mathrm{)}}_{\mathrm{2}}{\mathrm{PtCl}}_{\mathrm{6}}$ and ${\mathrm{(ND}}_{\mathrm{4}}{\mathrm{)}}_{\mathrm{2}}{\mathrm{PtBr}}_{\mathrm{6}}$ are tetragonal with a tentative space group ${\text{P4}}_2\mathrm{/n}$ and unit cell dimensions similar to those of the HTPs. The LTP of ${\mathrm{(ND}}_{\mathrm{4}}{\mathrm{)}}_{\mathrm{2}}{\mathrm{SeCl}}_{\mathrm{6}}$ may have an orthorhombic structure with a larger unit cell. The Rietveld refinement and Fourier analysis for the HTPs revealed that the D atoms in ${\mathrm{(ND}}_{\mathrm{4}}{\mathrm{)}}_{\mathrm{2}}{\mathrm{SeCl}}_{\mathrm{6}}$ and ${\mathrm{(ND}}_{\mathrm{4}}{\mathrm{)}}_{\mathrm{2}}{\mathrm{PtCl}}_{\mathrm{6}}$ are broadly distributed around the crystallographic threefold rotation axis with three weak overlapping maxima, while that of ${\mathrm{(ND}}_{\mathrm{4}}{\mathrm{)}}_{\mathrm{2}}{\mathrm{PtBr}}_{\mathrm{6}}$ is more strongly localized at three positions away from the threefold rotation axis. These results indicate that the transitions are due to orientational ordering of the ${\mathrm{ND}}_{\mathrm{4}}^{\mathrm{+}}$ ions and that the strong isotope effects may be quantum effects associated with the ${\mathrm{ND}}_{\mathrm{4}}^{\mathrm{+}}$ motion in a shallow rotational potential.