Relationship of the Absolute Compression Constant with Certain Physical Properties of Alkali Metal Halide Single Crystals
Abstract Generated abstract
This paper examines the absolute compression constant, defined as the molar volume of a compound divided by the sum of the molar volumes of its constituent elements, as a structural and physical parameter for alkali metal halide single crystals. Using tabulated data for lithium, sodium, potassium, rubidium, and cesium halides, the authors compare this constant with bond energy, breakdown voltage, refractive index, molecular refraction, entropy, thermal expansion coefficient, and heat of formation. The analysis indicates that, within series sharing the same alkali metal, increasing compression constant is associated with decreasing bond energy, electrical breakdown strength, and heat of formation, while refractive index, molecular refraction, entropy, and thermal expansion tend to increase. The paper argues that the relative ease of measuring this constant makes it a useful comparative quantity for studying how physical properties vary with chemical composition.
Full Text
RELATIONSHIP OF THE ABSOLUTE COMPRESSION CONSTANT
TO CERTAIN PHYSICAL PROPERTIES
OF ALKALI METAL HALIDE SINGLE CRYSTALS
V. V. POZDNYAKOV, A. A. BOTAKI
(Recommended for publication by the Department of General Physics, TPI)
The absolute compression constant \(C\) is defined as the quotient obtained by dividing the volume of one mole of a compound by the sum of the molar volumes of the components making up this compound.
I. I. Zaslavskii [1], having studied a large number of chemical compounds, came to the conviction that the absolute compression constant is an important property, closely connected with the structure and with many physical characteristics of a chemical compound. He established the following law: for a stable chemical compound, the molecular volume is approximately equal to half the sum of the molecular volumes of its constituent components. In 1856 D. I. Mendeleev [2] wrote: “...it is especially important to note that the volume of alkali oxides is less than the volume of the metal contained in them... For heavy metals the volume of the compound is always greater than the volume of the metal.”
N. N. Beketov [3] observed that, in the overwhelming majority of cases, a greater release of heat of formation also corresponds to a greater compression in the synthesis of a compound.
In accordance with Zaslavskii’s law, substances with \(C = 0.5\) are regarded as having normal compression.
Crystalline hydrates usually have a molecular volume that is too low in comparison with the sum of the atomic volumes of the components entering into the compound. In this case the compounds have \(C\) less than 0.5 and are regarded as having anomalously large compression; for example, \( \mathrm{HCl} + 2\mathrm{H}_2\mathrm{O} \) has \(C = 0.36\); \( \mathrm{KF} + 2\mathrm{H}_2\mathrm{O} \) gives \(C = 0.28\).
Unstable substances (often explosive ones) possess either very small compression or expansion upon formation of the compound. In analyzing the interrelation of the absolute compression constant with other physical properties of alkali metal halides, we used the values of the atomic volumes of chemical elements given in [1].
The table presented below contains the values of the absolute compression constants \(C\), the bond energies per ion pair \(U\), the breakdown voltage \(E_{\mathrm{br}}\), the absolute refractive indices \(n\), the molecular refractions \(R\), the entropies of the compound \(S\), the temperature coefficient of thermal expansion \(\alpha\), and the heats of formation \(Q\) for single crystals of alkali metal halides. On the basis of the data in the table it can be established quite definitely that, with an increase in the compression constant in a series of compounds with the same alkali metal
bond energy, breakdown voltage, and heat of formation decrease, while the refractive index, molecular refraction, entropy of the compound, and the temperature coefficient of linear expansion, on the contrary, increase. Thus, on the one hand, the absolute compression constant is closely interrelated with many physical properties of chemical
Table
| Compound | C | U, eV / pair of ions [4] | $E_{\mathrm{br}}$, MV/cm [4] | n [5] | R, cm³ [6] | S, cal/deg [7] | $\alpha \cdot 10^{-6}$, 1/deg [5] | Q, kcal/mol [3] |
|---|---|---|---|---|---|---|---|---|
| LiF | 0,43 | 10,56 | 3,1 | 1,38 | 2,31 | 9,7 | 30,7 | 145 |
| LiCl | 0,59 | 8,50 | — | 1,66 | 7,15 | 12,0 | 40,7 | — |
| LiBr | 0,68 | 8,05 | — | 1,78 | 9,89 | 15,7 | 46,7 | — |
| LiJ | 0,88 | 7,51 | — | 1,95 | 15,05 | 17,4 | 55,7 | — |
| NaF | 0,45 | 9,46 | 2,4 | 1,33 | 2,97 | 13,1 | 32,7 | 135 |
| NaCl | 0,57 | 7,93 | 1,5 | 1,53 | 8,29 | 17,5 | 36,7 | 99 |
| NaBr | 0,70 | 7,55 | 1,0 | 1,64 | 11,19 | 20,5 | 39,7 | 86 |
| NaJ | 0,83 | 7,07 | 0,8 | 1,79 | 16,21 | 22,5 | 46,5 | 65 |
| KF | 0,50 | 8,37 | 1,9 | 1,35 | 5,01 | 15,8 | 33,3 | 134 |
| KCl | 0,61 | 7,23 | 1,0 | 1,50 | 10,54 | 19,8 | 33,7 | 107 |
| KBr | 0,68 | 6,93 | 0,7 | 1,55 | 13,49 | 22,6 | 36,7 | 95 |
| KJ | 0,77 | 6,55 | 0,6 | 1,68 | 18,84 | 24,1 | 41,7 | 79 |
| RbF | 0,52 | 8,06 | — | 1,38 | 6,96 | 17,1 | — | — |
| RbCl | 0,61 | 6,95 | 0,8 | 1,49 | 12,21 | 22,0 | 32,8 | 106 |
| RbBr | 0,65 | 6,71 | 0,6 | 1,55 | 15,22 | 24,4 | 34,7 | 95 |
| RbJ | 0,77 | 6,34 | 0,5 | 1,64 | 20,72 | 26,3 | 39,7 | 83 |
| CsF | 0,46 | 7,6 | — | — | 9,34 | — | — | — |
| CsCl | 0,48 | 6,4 | — | — | 11,0 | — | — | — |
| CsBr | 0,52 | 6,2 | — | — | 18,34 | — | — | — |
| CsJ | 0,61 | 5,7 | — | — | 23,97 | — | — | — |
compounds; on the other hand, it is one of the physical quantities most accessible for measurement. This provides grounds for recommending the absolute compression constant as a very promising quantity that may be useful in studying the physical properties of compounds as a function of their chemical composition.
LITERATURE
-
I. I. Zaslavskii. Izv. Ivanovo-Voznesenskogo politekhnicheskogo instituta, Vol. 12, p. 61, 1918.
-
D. I. Mendeleev. Fundamentals of Chemistry. Vol. II, 12th ed. ONTI, Goskhimizdat, Moscow–Leningrad, 1934, p. 95; Gornyi zhurnal, No. 7, p. 1; SPB, 1856.
-
N. N. Beketov. Speeches of a Chemist. SPB, 1908.
-
A. A. Vorob’ev, E. K. Zavadovskaya. Izv. TPI, Vol. 83, p. 3, 1956.
-
P. A. Savintsev. Izv. TPI, Vol. 86, p. 216, 1958.
-
S. S. Batsanov. ZhNKh, Vol. 3, issue 2, 1958, p. 241.
-
K. E. Uiks, F. I. Blok. Thermodynamic Properties of 65 Elements, Their Oxides, Halides, Carbides, Nitrides. Handbook. Metallurgiya Publishing House, 1965.