A MODERN VIEW ON THE THERMOLISIS OF HYDRATE ZINC-MANGANESE(II) PHOSPHATES

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Introduction.
Solid solutions of phosphates of divalent metals, in particular zinc and manganese(II), are used as various construction materials with technically valuable properties that can be purposefully changed by varying the composition of the solid solution and its operating conditions [1,2].
Since most technological processes take place at elevated temperatures, in order to choose the optimal mode of operation of a solid solution, knowledge of its behavior during heating, composition, temperature intervals of formation and thermal There is no information on systematic studies of the thermal properties of a solid solution of zinc and manganese(II) hydrated phosphates in the literature.
The purpose of this work is to study the thermolysis of a solid solution of hydrated zinc and manganese(II) phosphates, to determine the composition, temperature intervals of formation and thermal stability of the products of its partial and complete dehydration.

Experimental
A solid solution of the general formula Zn 3-x Mn x (PO 4 ) 2 •4H 2 O (0<x≤1.00)(0<x≤1.00)was obtained by the interaction of a mechanical mixture of zinc (77.23 wt.% ZnO) and manganese (53.87 wt.% MnO) hydroxocarbonates with a solution of phosphoric acid (64.13 wt.% P 2 O 5 ) similar to that described in [2].A saturated solid solution -Zn 2 Mn(PO 4 ) 2 •4H 2 O and phosphates of different cation composition within the homogeneity of the solid solution were used as the main object of the study.
The behavior of phosphates during heating was studied in the temperature range of 25-900°С under dynamic conditions (derivatograph Q-1500D, platinum crucibles with a lid, standard -freshly calcined Al 2 O 3 , sample weight -300 mg, heating rate 0.6, 2.5, 5.0 and 10.0 degrees/ min.)and quasi-isothermal (labyrinth crucible, speed 3.0 degrees/min) heating modes.Heat treatment products obtained at temperatures corresponding to thermal effects on the DTA curve were identified similarly [3], using a complex of analysis methods: chemical, X-ray phase analysis, IR spectroscopy.
IR spectra were recorded at 20°C and -190°C in the range of 400-4000 cm-1 on a Nexus-470 spectrometer with Fourier transform and Omnic software.The samples were prepared by pressing a fixed amount (0.05 wt.%) into a KBr matrix.

Results and discussion
According to the results of a thermoanalytical experiment, zinc-manganese(II) phosphate of the composition Zn 2 Mn(PO 4 ) 2 .4H 2 O when heated at a rate of 2.5 degrees/min.thermally stable up to 100-105°С (Table ).Its thermal stability can be significantly increased by using a quasi-isothermal mode for heat treatment, in which  The spectrum of the sample obtained under the conditions of heating Zn 2 Mn(РO 4 ) 2 •4Н 2 O to 225°С is also completely similar to the previous one (Figure ).
The two absorption maxima recorded at 20°С in the ν(OH) region are not separated at  The IR spectrum of dehydrated at 900°С Zn 2 Mn(PO 4 )

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of partial and complete dehydration is necessary.

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in the results of modern scientific research 2023 August 2023 Conference proceedings 11 the partial pressure of water vapor above the sample is close to the thermodynamically balanced one.Under these conditions, mass loss in Zn 2 Mn(PO 4 ) 2 .4H 2 O tetrahydrate begins when heated to 135-145°С, in Zn 2.5 Mn 0.5 (PO 4 ) 2 •4H 2 O -to 130-135°С.

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This indicates that in the interval 190-225°С, the removal of the two least tightly bound water molecules takes place, which is accompanied by the rearrangement of the system of H-bonds in the structure of crystal hydrate.
AuthoringAn increase in the heat treatment temperature of Zn 2 Mn(РO 4 ) 2 •4Н 2 O above 100°С is accompanied by a loss of mass of the sample, which occurs in two stages, each of which corresponds to the removal of two water molecules.The first stage of dehydration occurs in the range of 105-190°С.The product formed at 190°С is identified as a dihydrate of the composition Zn 2 2 •4Н 2 O is similar to the spectrum of the sample obtained by heating it to 325°С.A clear splitting of absorption bands and an increase in their intensity are evidence of a more perfect structure of dehydrated phosphate.The obtained data complement the results of the X-ray analysis of the dehydration products of Zn 2 Mn(PO 4 ) 2 •4Н 2 O, according to which the completely dehydrated phosphate is identified as γ-Zn 2 Mn(PO 4 ) 2 .. It is stable when heated in the range of 325-900°C.Thus, thermolysis of zinc-manganese(II) phosphate of the composition Zn 2 Mn(РO 4 ) 2 •4Н 2 O is accompanied by thermal solid-phase transformations, the sequence of which can be represented by a general scheme: Mn(РO 4 ) 2 •4Н 2 O with a heating rate of 2.5 degrees/min.A change in speed leads to a shift in the temperature intervals of formation and thermal stability of both the original phosphate and the products of its partial and complete dehydration.So, at a heating rate of 0.6 degrees/min.Zn 2 Mn(РO 4 ) 2 •4Н 2 O is stable up to 90°С.In the range of 95-170°С (the first stage of water removal), a phosphate of lower hydration is formed -Zn 2 Mn(РO 4 ) 2 •2Н 2 O.The dihydrate is stable in the range of 190-215°С and loses 2 molecules of water with the formation of completely dehydrated γ-Zn 2 Mn(РO 4 ) 2 when heated to 295°С.Under conditions of a heating rate of 10.0 degrees/min.formation of Zn 2 Mn(РO 4 ) 2 •2Н 2 O and γ-Zn 2 Mn(РO 4 ) 2 is recorded at 115-235°С and 270-345°С, respectively.At the same time, the general regularities of the process are preserved.The given for Zn 2 Mn(РO 4 ) 2 •4Н 2 O dehydration scheme is correct for zinc-