Electrical and Thermal Properties of NH₄Br_xCI_1‐x Single Crystal

Abstract

A b s t r a c t . The effect of partial replacement of C1-ions by Brions in the mixed system of ammonium chloride and ammonium bromide during a set of thermal heating cycles is studicd. The study includes electrical measurements (d.e. resistivity, dielectric constant and the spontaneous thermal current), differential thermal analysis (DTA) and dilatometric thcrmal analysis in the tempera,ture range --6O"C up to 30°C. The results show a large change in the various measured parameters sensitive to thermal agitation process and to the presence of the bromine ions in the mixed crystal system close to the morphotropic boundary x == 0.5. At this boundary the temperature region of the ordered phase extended by about 28°C. The transformation enthalpy was reduced in such case to 41.3 cal/mol. According to the different experimental data of the system NH,Br,CI, --2, a phase diagram was proposed. This phase diagram predicted that the order-disorder phase transition point changed widely according to the value of x and retished -2°C when the mixed crystal system contained 50% of Br-ions. Elektrisehe und thermische Eigenschaften von NH~B1.,C11_,-EinkristallenI n h a l t s i i b e r s i c h t . Der Effckt einer teilweisen Ersetzung von C1--durch Br--Ionen in einem Alischsystem nus Ammoniumchlorid und Ammoniumbromid wjhrend eines Heizzyklusses wird elektrisch (Widerstand, dielektrische Konstante und spontaner Thermstrom), kalorimetrisch und dilatometrisch im Temperaturbereich von -60°C bis 30°C untersucht. Es zeigt sich ein groWer EinflnU thermischer Beweguiigsprozesse und der Bromkonzentration von 50%. Die Ergebnisse werden zu einem Phasendiugranim zusammengefaBt, das die Abhangigkeit des Ordnungs-Unordnnngsiibergangs von der Bromkonzentration unterstreicht. 1:)The values with asterisks are those obtained for ordered phase while the others for the disordered phase.

Rotationally ordered ammonium cations in the crystal structure of (NH4)2OsBr6

“…[9] In order to check for possible phase transitions, the largely unknown thermal behaviour of (NH 4 ) 2 OsBr 6 was studied. Heating in an open aluminium crucible in a furnace revealed beginning sublimation around 680 K. Order-disorder transitions in ammonium salts cause strong thermal effects (e. g. with a transformation enthalpy ΔH = 260 J / mol in NH 4 Br [17] ), which are easily detected by thermal analysis. Differential scanning (DSC, 113 K � T � 673 K) and thermogravimetry (TG, 293 K � T � 673 K), however, did not show any signals.…”