Jacob H. Greenberg

P-T-X phase equilibrium and vapor pressure scanning of non-stoichiometry in CdTe

Abstract Total vapor pressure was measured for all the three-phase equilibria in the CdTe system. P - T and T - X projections of the diagram were constructed. Results of the vapor pressure scanning of the CdTe solidus surface are reported. Tellurium non-stoichiometry is found to be up to (0.5–13) × 10 −3 at% at temperatures 945 to 1360 K while the cadmium non-stoichiometry is (0.1–4) × 10 −3 at% in the temperature range 695 to 1223 K. Position of the congruent sublimation curve S = V in the P - T - X phase space is discussed and some specifics of the solidus surface are outlined. The maximum congruent sublimation temperature was found to be 1324 K, 41 K lower than the maximum melting point of CdTe. Geometrical analysis of the phase equilibrium in the melting region of CdTe as well as compositions of the crystal, liquid and vapor are presented. Partial molar enthalpies and entropies of the component elements are deduced for CdTe.

Thermodynamic Properties of ZnTe in the Temperature Range 15–925 K

The heat capacity of ZnTe was measured in an adiabatic calorimeter in the temperature range from 15 to 330 K and by DSC at T = 290-925 K. The following standard thermodynamic properties of ZnTe were derived: C 0 p (298.15 K) = (49.52 ± 0.10) J K -1 mol -1 ; S 0 (298.15 K) = (81.94 ± 0.17) J K -1 mol -1 ; H 0 (298.15 K) - H 0 (0 K) = (10.98 ± 0.02) kJ mol -1 ; Φ 0 (298.15 K) = (45.12 ± 0.10) J K -1 mol -1 , Temperature dependences of the heat capacity and the Gibbs free energy function are derived for the temperature range up to 1500 K. The Debye temperature calculated from the specific heat data is Θ D = 252 K.

P–T–X Phase Equilibrium in the Zn–Te System

P-T-X data are the thermodynamic basis for modeling the crystal growth of materials with controlled stoichiometry. In this paper results of the first direct experimental measurement of the total vapor pressure are reported for three-phase equilibria SLV (solid-liquid-vapor) and VLS (vapor-liquid-solid) in Zn-Te, and the P-T projection of the P-T-X phase diagram is constructed. Geometrical analysis of the phase equilibrium showed that three congruent processes are observed in the Zn-Te system: congruent melting (S = L), sublimation (S = V), and vaporization (L = V). All three congruent curves are tangent to SLV. Consequently, all three congruent points are on the Te-side of the maximum melting point of ZnTe. The vapor pressure scanning method was applied to determine the maximum non-stoichiometry as a function of the temperature. The solidus volume of ZnTe was shown to be on the Te-side of the stoichiometric plane.

Topology of P-T-X phase diagrams of binary systems. Polymorphism, metastable states and high-pressure equilibria

Abstract A step-by-step geometrical approach is presented to the P-T-X phase equilibrium in binary systems with different types of polymorphism and metastable states, and the relationship between these two phenomena. Mutual solubility of the components in all the states of aggregation is taken into account, as well as formation of crystalline non-stoichiometric compounds. Special attention is given to equilibria at high pressures because until now, these types of phase diagrams have not been adequately discussed. At first, P-T relations in onecomponent systems are analysed and afterwards the third co-ordinate (composition X) is introduced for binary systems. The phase states of the systems are examined by means of P-T and T-X projections of the corresponding equilibria along with isobaric T-X sections of the representative and most complicated regions. All possible types of phase diagrams of binary systems with both parallel and non-parallel polymorphism of the components are discussed. Complete and limited solid state solubility is considered. Metastable states of the binary systems as well as different crystallization routes of non-stoichiometric compounds ( stable and metastable) are examined in connection with the polymorphism of the components. Certain general trends of metastable crystallization characteristic of all types of the phase diagrams are discussed

Sol-Gel preparation and characterization of fluorine substituted Bi(Pb)SCCO-2223 HTSC

Abstract Bi 2−x Pb x Sr 2 Ca 2 Cu 3 O y F 0.7 was prepared by sol-gel using citric acid and ethylene glycol. Thermodynamic calculations showed that a considerable loss of fluorine is to be expected. The samples were characterized by structural, thermal and physical methods. AAS showed that no appreciable amount of lead was lost during sintering. For x =0.35 the height Tc was observed, viz. 116K (onset). Using a selective fluoride electrode it was found that only up to 10% of the initial fluorine remained. The EPMA results showed that up to 90% of the sample contained (BiPb)SCCO-2223 the rest being (SrCa) 2 Cu 3 O y . Fluorine could not be detected by this method. Fluorine lead to significant enhancement of the phase formation.

Effect of fluorination on preparation of Bi(Pb)SCCO 2 2 2 3 by the citrate precursor process

Citrate precursor technology was used to prepare fluorine substituted Bi(Pb)SCCO 2 2 2 3 superconducting phase. Samples with the nominal composition of up to three F atoms per formula were synthesized. A number of experimental methods have been used to characterize the samples and to trace the phase transformations during the preparation process: DTA/TGA, XRD, EPMA (WDS and EDS), atomic absorption, potentiometry with fluoride selective electrode, resistance and inductive measurements. Fluorine was shown to enhance considerably the formation of the 2 2 2 3 phase. Thermodynamic calculations of P-T-X equilibrium in the Bi-Pb-Sr-Ca-Cu-O-F-C-H-N system were made in a wide temperature range to determine the composition of the vapours coexisting with the solid phases at different stages of the preparation process.

Experimental Data on P-T-X Phase Diagrams and Non-stoichiometry

II–VI semiconductor compounds are used for infrared, X-ray and γ-ray detection, in thin film solar cells, photo-refractive and blue/UV emission devices [118]. CdTe and Cd–Zn telluride, used as detectors for direct transformation of high-energy radiation to electrical signals, create a new generation of efficient detectors for medical applications, such as tomography. CdZnTe is used as a buffer layer in heteroepitaxy of mercury-cadmium telluride and as a substrate for epitaxial technology of (Hg,Cd)Te, the most important semiconductor material for infrared detector applications [119]. The next generation of infrared detection devices demands further improvement in material technology. These applications require high-quality multilayer heteroepitaxial structures with buffer layers serving to overcome the adverse effects of lattice mismatch, in particular, between Si or GaAs substrates and the active II–VI layer [120]. The lattice constant of (Cd,Zn)Te can be adjusted by changing the content of ZnTe to match the lattice of the (Hg,Cd)Te epilayer [121]. Some of the applications of II–VI bulk single crystals, according to P.Rudolph [122], are presented in Table 2.

Experimental Methods of Investigating P-T-X Phase Equilibrium

Experimental study of P-T-X phase equilibrium consists of determining the relationships between the temperature, pressure, and composition of the phases that are involved in a particular equilibrium. According to the phase rule, two-phase equilibrium in a binary system is fixed by two independent parameters and in three-phase equilibrium only one parameter can be changed independently without disturbing the state of the system. Consequently, a complete study of the P-T-X phase diagram involves measuring the functional dependences P = P(T,X), T = T(X), and P = P(T).

Vapor-pressure scanning of solids. A novel way to probe non-stoichiometry

Abstract Investigation of non-stoichiometry of important classes of inorganic materials is often hampered by a very narrow (