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Abstract. We report on the growth and initial thermal and optical characterisation of a single-crystalline ternary halide, Tl 4 CdI 6 . The crystal is described by the centrosymmetric tetragonal class P4/mnc at the room temperature. The parameters of linear thermal expansion are studied in the temperature region 330-500 K. The crystal is optically positive and reveals very high optical anisotropy (the birefringence ~ 0.13 at 633 nm). Its optical pseudo-gap value corresponds to widegap semiconductors. A broad maximum at 400-410 K found in the temperature dependence of IR radiation-induced second harmonic generation may be indication of a polymorphic phase transformation.

Íà îñíîâ³ ñàìîóçãîäaeåíèõ ðîçðàõóíê³â ìåòîäîì ïñåâäîïîòåíö³àëó ïðîâåäåíî òåîðåòè÷íå âèâ÷åííÿ ñòðóêòóðíèõ ³ åëåêòðîííèõ âëàñòèâîñòåé ìîíîêðèñòàë³â InI ó øèðîêîìó ä³àïàçî-í³ çì³íè çîâí³øíüîãî òèñêó. ϳäòâåðäaeåíî âèçíà÷àëüíó ðîëü íàäëèøêîâî¿ 5s 2 åëåêòðîííî¿ ïàðè ³îíà In + ó ôîðìóâàíí³ ñòðóêòóðè òà ñòàá³ë³çàö³¿ ñïîëóê, ùî ì³ñòÿòü êàò³îíè ³ç çíèaeåíîþ âàëåíòí³ñòþ. Âèçíà÷åíî çíà÷åííÿ ãðàíè÷íîãî òèñêó ôàçîâîãî ïåðåõîäó InI ó âèñîêîñèìåò-ðè÷íèé ñòðóêòóðíèé òèï CsCl. Âñòàíîâëåíî, ùî ïåðåä ñòðóêòóðíèì ïåðåõîäîì â³äáóâàºòüñÿ ïåðåõ³ä íàï³âïðîâ³äíèê-ìåòàë. ϳñëÿ ñòðóêòóðíîãî ïåðåõîäó ñïîëóêà çàëèøàºòüñÿ íàäàë³ êâàç³îäíîì³ðíèì ìåòàëîì.

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