On the Structural Transition from NaCl to CsCl Type in Alkali Hydrides

Abstract

The solid sphere ion model is used to compare specific volumes of NaCl and CsCl phases of alkali hydrides. The transition of the KoCl to CsCl type is shown to be thermodynamically possible in K, Rb, and Cs hydrides. A phase transition is detected in K H a t pressures about 40 kbar, and the T-P diagram of K H is studied. On the basis of the analysis performed, the NaCl to CsCl transition in LiH is unlikely, unless the electron configurations of the ions are changed. B npH6JIHWeHHH WeCTKHX C@ep IIpOBeHeHO CpaBHeHIIe YneJIbHbIX 06' beMOB NaCl II CSCl MOaH@HKZU&i rHDpHnOB ILIeJIOYHbIX MeTaJIJIOB. nOKa3aH0, 9TO B rHHpHJIaX I<, Rb H C S non EasneHneM TepMoipiHa;nHrecmi B O~M O S~C~H nepexon m n a NaCI-CsCI. IIpn nasneHam OKOJIO 40 kbar 0 6~a p y m e~ nepexoa H HccnenoBaHa IT-P HHarpaMMa rxnpmna KanHfi. Ha ocHoBaHm nponenaHHoro a~a n m a Pu' aC1-CsC1 nepexon B LiH npencTaBnfieTcfi Mano-BepOfiTHbIM, eCjIH IIpH IIepeXORe He H3MeHHTCJl3JIeHTpOHHafi €iOH@lHfypallHH HOHOB.

High Pressure X-Ray Investigation of the Alkali Hydrides NaH, KH, RbH, and CsH*

“…This has already been suggested by Bashkin et al [1] based on the assumption that the alkali hydrides should show a similar behaviour as the alkali halides. This has already been suggested by Bashkin et al [1] based on the assumption that the alkali hydrides should show a similar behaviour as the alkali halides.…”

“…Bashkin et al [1] have pointed out, that as in the case of most alkali halides with NaCl-type structure, the alkali hydrides may also undergo a structural phase transition to a CsCl-type structure when high pressure is applied. Bashkin et al [1] have pointed out, that as in the case of most alkali halides with NaCl-type structure, the alkali hydrides may also undergo a structural phase transition to a CsCl-type structure when high pressure is applied.…”

Metal Hydrides under High Hydrostatic Pressure

“…This tendency is similar to that observed for alkali-metal halides. 22,28 Data of Hochheimer and Bashkin were in relatively good agreement. Hochheimer et al, in spite of much higher range of available pressure (28 GPa) also were unable to trace the B1-B2 transition in NaH but derived the value of bulk modulus for B1 phase of this hydride.…”

“…It has been found that at high pressure the B1-type halides transform into B2-type structure. [19][20][21] Following this observation Bashkin et al 22,23 measured volume of potassium and cesium hydrides as a functions of pressure. In these experiments they used tungsten carbide piston-cylinder device so the in situ determination of structural changes was not possible.…”

“…However, they could confirm transition from B1 (NaCl-type) into the B2 (CsCl-type) structure by low temperature XRD measurements of samples taken from high pressure device after preliminary quenching. By using a simple hard-sphere ion model and assuming similar behavior of alkali-hydrides and alkali-halides Bashkin et al 22 have predicted that B1-B2 transition is thermodynamically possible for all alkali-metal hydrides (excluding LiH) and proved this prediction experimentally (but not in a direct way) for KH 22 , CsH 23 and RbH. 24,25 The volume changes during B1-B2 transitions for these hydrides were 8.5 % for CsH 23 and 10.6 % for RbH 24 .…”