Химия и химические технологии/7. Неорганическая химия

Bekturganova A.Zh.¹, Rustembekov K.T.¹, Stoev M.²,

Ybrahym S.A.¹, Abdrahmanova D.S.¹, Toyganbaeva D.Zh.¹

¹Ye.A.Buketov Karaganda State University, Kazakhstan

²South-West University «Neofit Rilski», Bulgaria

Calorimetry of double zinc tellurite

Knowledge of the thermodynamic properties of complex oxo compounds is necessary to create a data bank of thermodynamic quantities of data, modeling of processes of synthesis of new materials with desired characteristics and to identify the fundamental dependence "structure - property" in the synthesized substances.

The aim of this work was calorimetric research of the heat capacity of double zinc tellurite. Tellurite composition Na₂Zn(TeO₃)₂ was synthesized by ceramic technology from tellurium (IV) oxide, zinc and sodium carbonate. The formation of the equilibrium composition of the compound was monitored by X-ray diffraction.

The heat capacities of tellurite were studied via dynamic calorimetry on an commercial IT-S-400 calorimeter in the temperature range of 298.15-673 K (Table 1). Errors in the measurement of the heat capacity at all temperatures are within the accuracy of the instrument (± 10%).

Table 1

Experimental specific and molar heat capacities of the Na₂Zn(TeO₃)₂

Т, К	, J/(g К)	, J/(mol К)	Т, К	, J/(g К)	, J/(mol К)
1	2	3	4	5	6
298,15	0,4683±0,0162	217±21	498	0,6892±0,0043	319±6
323	0,5230±0,0045	242±6	523	0,6969±0,0052	322±7
348	0,6038±0,0039	279±5	548	0,5827±0,0097	270±13
373	0,4183±0,0108	193±14	573	0,6318±0,0030	292±4
1	2	3	4	5	6
398	0,4972±0,0077	230±10	598	0,6689±0,0043	309±6
423	0,5455±0,0065	252±8	623	0,6947±0,0071	321±9
448	0,6057±0,0086	280±11	648	0,7203±0,0110	333±14
473	0,6312±0,0067	292±9	673	0,7358±0,0051	340±7

For an average of the specific heat at each temperature was determined by standard deviations (), and for molar heat capacities - random error components . Random error components experimental values of heat capacities do not exceed the limits of the instrument error. Operation of the calorimeter was tested by measuring the heat capacity of a-Al₂O₃. The resulting (298,15) = 76.0 J/(mol К) was in satisfactory agreement with the reference value (79.0 J/(mol К)).

In studying the temperature dependences of the heat capacities of Na₂Zn(TeO₃)₂ was at 348 and 523 K, we observed abnormally sharp l-like peaks probably associated with second-order phase transitions (Figure). Тhese transitions could be due to the redistribution of cations, changes in coefficient of thermal expansion and changes in the magnetic moment of synthesized tellurite.

Т, К

Figure - The temperature dependence of heat capacity of tellurite Na₂Zn (TeO₃)₂ in the range of 298.15 - 673 K

Due to the presence of second-order phase transitions, the dependence of compound described by several equations the coefficients of which are given in Table 2.

Table 2

Equations of the temperature dependence of heat capacity of the

Na₂Zn (TeO₃)₂ in the range of 298.15 - 673 K

Compound	Coefficients of Equation , J/(mol К)			, К
	a	b·10^-3	c·10^5
Na2Zn(TeO3)2	–156,8±5,5	1253,0±44,0	–	298-348
	1473,8±51,7	–3432,4±120,5	–	348-373
	682,3±24,0	–334,0±11,7	–506,7±17,8	373-523
	1427,3±50,1	–2112,8±74,2	–	523-548
	918,1±32,2	–477,1±16,8	–1162,6±40,8	548-673

Using the experimental data on , and the estimated values of , from the known ratio in the range of 298.15 – 673 K we calculated the temperature dependence of functions , , , (Table 3).

Table 3

Thermodynamic functions of the Na₂Zn (TeO₃)₂

in the range of 298.15 - 673 K

Т, К	, J/(mol К)	, J/(mol К)	, J/mol	, J/(mol К)
1	2	3	4	5
298,15	217±8	288±9	–	288±19
300	219±8	289±19	436±15	288±19
325	250±9	308±20	6306±221	289±19
1	2	3	4	5
350	282±10	328±21	12960±455	291±19
375	187±7	344±22	18698±656	294±19
400	232±8	356±23	24077±845	296±19
425	260±9	372±24	30236±1061	301±20
450	282±10	387±25	37016±1299	305±20
475	299±11	403±26	44283±1554	310±20
500	313±11	420±27	51938±1823	316±21
525	323±11	434±28	59887±2102	320±21
550	265±9	448±29	67179±2358	326±21
575	292±10	461±30	74233±2606	331±22
600	309±11	473±31	81754±2870	337±22
625	322±11	486±32	89651±3147	343±22
650	333±12	499±32	97846±3434	349±23
675	341±12	512±33	106273±3730	354±23

Thus, the isobaric heat capacities of double zinc tellurite were determined by dynamic calorimetry in the temperature range 298.15 - 673 K. Equations describing their temperature dependences were derived. Dependences for tellurite Na₂Zn(TeO₃)₂ at 348 and 523 K were found to have sharp abnormal peaks, l-like effects attributable to a second-order phase transition. Thermodynamic functions , , , were calculated. The existence of a second-order phase transition in the curve of the temperature dependence indicates that this compound could have unique electrophysical properties.