Congruent Vaporization of Solid Manganese Monotelluride and the Effects of Phase Transitions:  A High-Temperature Mass Spectrometric Study

Abstract

Vaporization studies on Mn-Te samples of initial compositions 49.6 and 54.9 at. % Te were conducted by Knudsen effusion mass spectrometry. Both these samples on continuous vaporization reached the congruently effusing compositions (CECs) ≈ 50 at. % Te. Vaporization chemistry around this composition was studied from 1194 to 1343 K to examine the effects associated with the reported phase transitions in MnTe(s) at 1228 (R T β), 1293 (β T γ), and 1323 K (γ T δ). Among these, only the R T β phase transition showed significant effects. During the phase transition, the vapor phase was relatively richer in manganese in the increasing temperature direction (i.e., as the sample was heated from T e 1203 K to T g 1238 K) and was relatively richer in tellurium in the decreasing temperature direction (i.e., as the sample was cooled from T g 1238 K to T e 1218 K). Evidence for a slight variation in the CEC with temperature was also obtained but was distinguishable from the effects associated with the R T β phase transition. From the results obtained in four series of experiments, the partial pressures of Mn(g), Te(g), and Te 2 (g) were deduced, and by neglecting the variations in the CECs with temperature, the enthalpy changes for the following vaporization reactions during congruent effusion were evaluated: MnTe(s) ) Mn(g) + 0.5Te 2 (g), ∆ r H°m(298.15K) ) 463.6 ( 2.1 kJ mol -1 ; MnTe(s) ) Mn(g) + Te(g), ∆ r H°m(298.15K) ) 592.4 ( 2.3 kJ mol -1 ; MnTe(s) ) Mn (g) + yTe(g) + (1y)/2Te 2 (g), ∆ r H°m (298.15 K) ) 554.0 ( 4.6 kJ mol -1 , y mean ≈ 0.7. These results yielded ∆ f H°m(MnTe,s,298.15K) ) -99.2 ( 6.8 kJ mol -1 . An equation for the total vapor pressure during congruent effusion of MnTe(s), valid for the temperature range from 1194 to 1343 K was obtained: log(P/Pa) ) -(14539 ( 273)/(T/K) + (11.159 ( 0.222).

Vaporization studies on (manganese + tellurium) withx(Te) > 0.5 by Knudsen effusion mass spectrometry

“…(20) In the case of MnTe, assigning the stoichiometric composition for its Te-rich boundary (c = 1) at 650 K < T < 750 K may not be seriously incorrect, if we draw upon an inference from our previous study (9) of its congruently effusing composition (CEC): with decrease in temperature, the x(Te) corresponding to the CEC would decrease from the value of (0.492 ± 0.011) determined at 1194 K T 1223 K, and the Te-rich boundary, not far from it, may well have x(Te) ≈ 0.5. Nevertheless, we chose x(Te) = 0.507, which corresponds to a value of c = 1.03 deduced from the results given by Abrikosov et al (17) and Furberg.…”

“…As part of our program to obtain such information, we have conducted high-temperature mass spectrometric investigations on tellurides of Fe, (1,2) Cr, (3,4) Ni, (5) Mo, (6,7) and Mn. (8,9) In the (manganese + tellurium) system, our previous studies both revolved around the manganese monotelluride phase: determination of the composition and thermodynamic properties corresponding to the Mn-rich phase boundary; (8) and investigation of vaporization behavior during congruent vaporization and condensed-phase transitions. (9) The present work concerns manganese ditelluride, the only other phase known to exist in the (manganese + tellurium) system.…”

“…(8,9) In the (manganese + tellurium) system, our previous studies both revolved around the manganese monotelluride phase: determination of the composition and thermodynamic properties corresponding to the Mn-rich phase boundary; (8) and investigation of vaporization behavior during congruent vaporization and condensed-phase transitions. (9) The present work concerns manganese ditelluride, the only other phase known to exist in the (manganese + tellurium) system. It consists of measurements of vapor pressure over the (manganese monotelluride + manganese ditelluride) two-phase region, and determination of phase boundaries and thermodynamic properties of the MnTe 2 phase.…”

Effects of condensed-phase transitions in congruently vaporizing systems

“…The mass spectrometric investigation by Lakshmi Narasimhan et al (12) confirmed that the atomic fractions of Mn and Te in the condensed phase are constant, and thus vaporization is congruent. During congruent vaporization,…”

“…Such effects are better understood now after sustained research on this topic. (3)(4)(5)(6)(7)(8)(9)(10)(11)(12) Suppose a congruently vaporizing two-component condensed phase A x B (1−x) (CP1) transforms upon a temperature change to another congruently vaporizing condensed phase A y B (1−y) (CP2) within an effusing or transpiring system in which the rate of vapor loss is arbitrarily slow such that equilibrium obtains. For a three-phase equilibrium involving vapor, CP1, and CP2, the conservation of matter requires that compositions of all the three phases be different.…”