Phase transition and freezing of disordered ionic orientation in CsNO2 crystal

✍ Scribed by Keiichi Moriya; Takasuke Matsuo; Hiroshi Suga

• Publisher: Elsevier Science
• Year: 1981
• Tongue: English
• Weight: 420 KB
• Volume: 82
• Category: Article
• ISSN: 0009-2614
• DOI: 10.1016/0009-2614(81)85445-0

✦ Synopsis

Heat capactty anomalies due to phase transitron and glass transttion were found at 209.16 + 0.10 K and around 41 K. Dielectric reLt..ation 1ta.s obsellred between 58 and 130 K. An anomalous effect, probably related to the orientational dynamics of the NO; ion, was found in laser-Raman spectra of the bending -vibration of the anion I _ Introduction Alkali metal and thallium nitrites form an mteresting group of ionic crystals which undergo phase transitions arising from the configurational disorder of the mtrite ions [l-5] _ Cesium nitrite, CsN02, crystallizes in the cesium chloride structure (space group 0: Pm3m) at ambient temperature [2]_ The V-shaped NOT ion must be orientationally disordered in view of the octahedral symmetry of the anion site. A phase transrtron is thus expected in this crystal. In fact, Richter and Pistonus [2] have reported that CsN02 transforms at 179 K to a low-temperature rhombohedral phase (space group Rjm)_ The heat capacity of CsNO, above liquid-nitrogen temperature was measured by Staveley and Mraw [3] _ They found a phase transition at 208.85 f 0.10 K which probably corresponds to the transition found in the high-pressure work [3]. The symmetry of the low-temperature phase [2 J requires that orientational disorder of the nitrite ions persists in this phase. This gives rise to the possibrlity of another phase transition below 78 K or freezing of the disorder into an immobile state. Many works were reported on the phase transition of KCN and NaCN crystals m relation to the orienta-' Contribution No. 21 from Chemical Thermodynamics