86 M. V. Rotermela, T. I. Krasnenkoa, S. A. Petrovab, S. G. Titovab a Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences, 91 Pervomayskaya St., Ekaterinburg, 620990, Russian Federation b Institute of Metallurgy, Ural Branch of the Russian Academy of Sciences, 101Amundsena St., Ekaterinburg, 620016, Russian Federation rotermel@ihim.uran.ru Conditions for the formation of a non-autonomous phase at the structural deformation of complex vanadium oxides A new previously unknown effect of a reversible transition from a single- phase system to a heterophase system containing a non-autonomous phase was observed during thermal and chemical deformations of the Zn 2–2x Cd 2x V 2 O 7 structure.The role of local symmetry in the formation of the non-autonomous phase is shown on the basis of X-ray diffraction studies in situ and a comparative crystal-chemical analysis of the structural deformations of isoform monoclinic solid solutions of zinc and copper pyrovanadates with zero volumetric thermal expansion. Keywords: non-autonomous phase, volume thermal expansion, M 2 V 2 O 7 Received: 02.03.2018. Accepted: 10.04.2018. Published: 10.05.2018. © Rotermel M. V., Krasnenko T. I., Petrova S. A., Titova S. G., 2018 D O I: 1 0. 15 82 6/ ch im te ch .2 01 8. 5. 1. 06 Introduction The non-autonomous phase is a phase whose existence under a certain ther- modynamic conditions is possible only in an ensemble with one or more compounds. Since the introduction of R. Defay in 1961 [1, 2], the term of “non-autonomous phase” was used, primarily, in connection with the assumption of a special role of the surface where the concentration of defects is higher than in the volume. To date, non-autono- mous phases have been detected as com- ponents of various heterophase systems. Thus, considerable attention has been paid in recent years to non-autonomous surface phases in the geological and mineralogical literature, where their roles in the adsorp- tion and segregation of impurities on the surface, or in interphase and intergranular borders, and also in the transfer of matter from high pressure to normal conditions have been established [3–7]. Stabilization of such phase associations is provided by an increase in the total free energy due to the energy of coherent inter-phase boundaries. Changes in the elemental composition of the surface of synthetic single crystals and ceramics due to thermally activated he- terosegregation were shown earlier [8]. A supramolecular concept of eutectics that takes into account the interaction of dis- proportionate substructures and reveals mechanisms of formation of supramo- Rotermel M. V., Krasnenko T. I., Petrova S. A., Titova S. G. Chimica Techno Acta. 2018. Vol. 5, No. 1. P. 86–91. ISSN 2409–5613 87 lecular ensembles in boundary layers, i.e. non-autonomous phases, was suggested in a number of works, which studied eu- tectic alloys in inorganic systems [9]. A “compo site effect” resulting in high ionic conducti vity was found in several binary systems, and has been used for prepara- tion of electrode materials for the chemical power sources because of the high ionic conductivity of the particles of the non- autonomous phase. The participation of non-autonomous phases in the solid-phase synthesis and sintering of ceramic sam- ples at Tammann temperature due to the melting of the non-autonomous phase on the matrix surface were assumed and ther- modynamically justified, as well as par- ticipation of the non-autonomous phase in the charge and mass transfer processes was shown [10–16]. We discovered a new, previously unknown effect of a reversible transition from a single-phase system to a heterophase system; the latter, together with the main phase, contains also the non-autonomous phase (NP). Monoclinic β-Zn2–2xCd2xV2O7 solid solutions (space group C2/m) contain the non-autonomous phase within the temperature range 20– 735 °C with the compositional range varied from 2.5 to 50 mol.% of Cd2V2O7 in amount of several percent (Fig. 1). The increase of temperature narrows the region of the heterophase system. High-temperature X-ray diffraction me- thod showed that negative or close to zero volume thermal expansion was observed inside the region where β-Zn2–2xCd2xV2O7 and the non-autonomous phase coexisted. A precise high-temperature and low-tem- perature in situ X-ray diffraction studies of thermal expansion for a number of monoclinic β-Cu2–2xZn2xV2O7 (x  = 0.15, 0.3, 0.4, 0.6) solid solution samples (space group C2/c) were performed in order to set the conditions for appearance and sta- bilization of the non-autonomous phase. Although the negative volume thermal ex- pansion over a wide temperatures range was observed, the presence of the non- autonomous phase in the entire region of Cu2–2xZn2xV2O7 existence was not con- firmed (Fig. 1). A comparative analysis of two vanadate systems that contain the solid solutions with identical chemical formula and monoclinic crystal structure, both with the wide range of close to zero and nega- tive volume thermal expansion, allows to propose the structural model for the ap- pearance and stabilization of the non-au- tonomous phase for the low-symmetric pyrovanadates of divalent metals. Experimental Solid solutions in the Zn2V2O7– Cd2V2O7 and Zn2V2O7– Cu2V2O7 systems were obtained by the ceramic synthesis method. X-ray in situ studies were carried out using a diffractometer SHIMADZU XDR7000 (Cu Kα radiation) in the range of angles 2θ from 5° to 120° with steps 0.02°, with silicon as an external standard. High-temperature measurements were performed with an Anton Paar TTK-450 attachment. The crystal structure refine- ment of the powder diffraction data was made using the GSAS software. Results and discussion The changes of unit cell parameters for the solid solutions within the temperature range from the room temperature up to 800 °C were analyzed (Fig. 2) in order to 88 establish the relations between the ap- pearance of non-autonomous phase in the Zn2V2O7 – Cd2V2O7 system and the transformations of the matrix phase. Maxi- mum thermal deformations, accompanied by the close to zero volume thermal expan- sion (for β-Zn1.6Cd0.4V2O7 αV = –0.08×10 –5 deg-1), were observed within the region of β-Zn2–2xCd2xV2O7 and the non-autonomous phase coexistence. Noticeable expansion of monoclinic solid solution was observed out of the range of stabilization for the non- autonomous phase. Significant changes of the unit cell parameters a and b take place inside the region of heterophase system. The maximal transformation of the mono- clinic plane (ac) happens due to the change in the β angle, which causes shear deforma- tion at a fixed rotation angle of [VO4]-tetra- hedron; V – Obridge–V = 180°, C2/m (Fig. 3). Analysis of the changes in the unit cell pa- rameters for the β-Zn2–2xCd2xV2O7 solid solutions indicates the existence of a cor- relation between the appearance and dis- appearance of the non-autonomous phase and the shear strain intensity with a change Fig. 1. Phase equilibria in the Cd2V2O7– Zn2V2O7 and Zn2V2O7– Cu2V2O7 systems:  – solid solution Cd2–2xZn2xV2O7;  – solid solution of β-Zn2–2xCd2xV2O7;  – solid solution β-Zn2–2xCd2xV2O7+ NP;  – solid solution of α-Zn2–2xCd2xV2O7;  – solid solution α-Zn2–2xCu2xV2O7;  – solid solution of β-Zn2–2xCu2xV2O7;  – solid solution β-Cu2–2xZn2xV2O7; – solid solution of β’-Cu2–2xZn2xV2O7;  – solid solution α-Cu2–2xZn2xV2O7; – solidus line Fig. 2. The unit cells parameters for the β-Zn2–2xCd2xV2O7 solid solutions versus temperature β-Zn2–2xCd2xV2O7 Cd2V2O7 Т, °C Zn2V2O7 100 200 300 400 500 600 700 800 900 1000 Cu2V2O7 0 200 400 600 800 285 288 291 107,2 108,0 108,8 109,6 9,99 10,02 10,05 10,08 8,46 8,52 8,58 8,64 7,00 7,04 7,08 7,12 V , Å 3 t , o C β, o c, Å b, Å Zn 1 , 75 Cd 0, 2 5 V 2 O 7 Zn 1 , 63 Cd 0, 3 7 V 2 O 7 Zn 1 , 6 Cd 0 ,4 V 2 O 7 Zn 1 , 2 Cd 0 ,6 V 2 O 7 a , Å 89 in the monoclinic angle. Significant shear deformations were detected exactly in the region where β-Zn2–2xCd2xV2O7 and non- autonomous phase coexisted; therefore, it can be assumed that the monoclinic plane in this case serves as a substrate generating the non-autonomous phase. An increase in temperature or concentration of dopant cation leads to the disappearance of the shear deformation, and at the same time – to the disappearance of the non-autono- mous phase, as shown in Fig. 1. Thermal behavior of two representative solid solution samples in the Zn2V2O7 – Cu2V2O7 system, Cu1.6Zn0.4V2O7 and Cu0.8Zn1.2V2O7, were studied in a wide temperature range (–180–400 °C) in or- der to determine the dependence of the non-autonomous phase appearance on the nature of thermal deformations of crystal lattice. The temperature dependencies of the unit cell parameters for Cu1.6Zn0.4V2O7 and Cu0.8Zn1.2V2O7 are shown in Fig. 4. One can see that the values of unit cell parameters change monotonously within the studied temperature range. The a para- meter decreases significantly with tempera- ture. The values of negative volume ther- mal expansion of the lattice for the both solid solutions correlate well with changes in the parameter a (for Cu1.6Zn0.4V2O7 αV = –3.14×10–5 deg-1). The parameter c and the monoclinic angle β remain practically unchanged. It should be noted that the rotation angle of vanadium-oxygen tet- rahedron V – Obridge – V for the samples Cu1.6Zn0.4V2O7 and Cu0.8Zn1.2V2O7 at tem- perature –180 °C was equal to 131.6(8)° and 129.9(7)°, respectively, and slightly decreased to 130.5(4)° and 129.3(4)°, re- spectively, when temperature was raised to 400 °C, since the structure belongs to the space group C2/c. Based on the ob- tained data, one can conclude that the ab- sence of the non-autonomous phase in the Zn2V2O7 – Cu2V2O7 system is due to the combined effect of two factors: insignificant shear deformations of the crystal lattice, governed by the change of the monoclinic angle β, and the mobility of vanadium-oxy- gen subsystem (Fig. 5) due to additional turn of vanadium-oxygen bi-ortogroups, Fig. 3. The monoclinic angle and the rotation angle of [VO4]-tetrahedron in β-Zn2–2xCd2xV2O7 vs temperature 0 100 200 300 400 500 600 165 170 175 180 185 190 195 -Zn1 ,6Cd0 ,4V2O 7, V -O-V V - O - V , o t, oC 107,7 108,0 108,3 108,6 108,9 -Zn 1,6 Cd 0 ,4 V 2 O 7 , = -1.82 10 -5 1/deg , o β β β αβ Fig. 4. The unit cells parameters for the Cu1,6Zn0,4V2O7 and Cu0,8Zn1,2V2O7 solid solutions vs temperature -200 -100 0 100 200 300 400 576 580 584 588 109,6 110,4 111,2 10,02 10,08 10,14 8,04 8,10 8,16 8,22 7,44 7,52 7,60 7,68 t, o C V , Å 3 , o c, Å b, Å 80% Cu 2 V 2 O 7 - 20% Z n 2 V 2 O 7 40% Cu 2 V 2 O 7 - 60% Z n 2 V 2 O 7 a, Å β 90 since the angle of the V – Obridge – V is not fixed by the symmetry rules. A comparative cr ystal-chemical analysis of monoclinic solid solutions β-Zn2–2xCd2xV2O7 and β-Cu2–2xZn2xV2O7 showed that the shear deformations are more intensive for β-Zn2–2xCd2xV2O7 in the temperature range from the room tem- perature up to 600  °C (–1.2×10–5 deg–1), which correlates with the appearance of the non-autonomous phase. In the case of β-Cu2–2xZn2xV2O7 (0.42×10 –5 deg–1), where shear deformations are absent, no non-au- tonomous phase was detected. Thus, the assumption that non-autonomous phase is generated by the shear deformations of the matrix phase due to a change in the monoclinic angle was confirmed. Conclusion The set of in situ X-ray diffraction data and a comparative crystal-chemical analy- sis of zinc and copper pyrovanadates solid solutions that take into account thermal and chemical deformations of the struc- ture allowed to established that the non- autonomous phase is formed when the following conditions are achieved: (1) the volume thermal expansion is close to zero; (2) shear deformations due to a change in the monoclinic angle appear (the degree of freedom that is not fixed by the sym- metry rules in the monoclinic crystals); (3) compounds belong to the space group C2/m (mirror symmetry, the angle of V – Obridge – V chain is 180°). The appearance of the non-autonomous phase under these conditions is a result of the matrix phase adaptation to the increase of inner-crys- talline pressure that arises, while the unit cell volume remain unchanged, because of the shear deformation and a symmetry prohibition for the polyhedron architecture changes. Acknowledgements The work was supported by UB RAS (project 18-10-3-32). References 1. Rossini FD, Prigogine I. Chemical Thermodynamics. Wiley; 1961. 514 p. 2. Defay R, Prigogine I, Bellemans A. Surface Tension and Absorption. London: Long- mans, Green; 1966. 431 p. 3. Urusov VS, Tauson VL, Akimov VV. Geokhimiya tverdogo tela [Geochemistry of solid state]. Мoscow: GEOS, 1997. 500 p. Russian. 4. Schegol’kov YuV, Tauson VL, Medvedev VYa, Pochekunina MV, Ivanova LA, Lipko SV. Vzaimodeystvie poverkhnosti elementnogo zolota s flyuidami – klyuch k ponimaniyu mekhanizmov perekondensatsii i mobilizatsii zolota v endogennykh i ekzogennykh usloviyakh. Doklady RAN. 2007;412(6):810–3. Russian. Fig. 5. The monoclinic angle and the rotation angle of [VO4]-tetrahedron in Cu2–2xZn2xV2O7 versus temperature - 200 -1 00 0 100 2 00 30 0 40 0 1 29,3 1 29,6 1 29,9 1 30,2 1 30,5 1 30,8 1 31,1 1 31,4 1 31,7 -Cu1, 6Zn0 ,4V2O 7, V -O-V = -1 .57 10 -5 1 /de g -Cu0, 8Zn1 ,2V2O 7, V -O-V = -0 .79 10 -5 1 /de g V -O -V , o t , o C 10 9 ,8 11 0 ,1 11 0 ,4 11 0 ,7 11 1 ,0 11 1 ,3 11 1 ,6 11 1 ,9 - Cu 1,6 Zn 0, 4 V 2 O 7 , = 0 .44 1 0- 5 1/deg - Cu 0,8 Zn 1, 2 V 2 O 7 , = 0 .34 1 0- 5 1/deg , o β β β β β αβ αβ α α 91 5. Lipko SV, Tauson VL, Akimov VV, Zel’berg BI, Knizhnik AV. Povedenie elementov- primesey v poroshkovom alyuminii [Behavior of elements-admixtures in powdered aluminium]. Tsvetnaya Metallurgiya [Non-ferrous Metallurgy]. 2006;4:13–9. Russian. 6. Tauson VL, Kravtsova RG, Smagunov NV, Spiridonov AM, Grebenschikova VI, Budyak AE. Strukturnoe i poverkhnostno-svyazannoe zoloto v piritakh mestorozh- deniy raznykh geneticheskikh tipov. Doklady RAN. 2006 ;406(6):806–9. Russian. 7. Tauson VL, Kravtsova RG, Smagunov NV, Spiridonov AM, Grebenshchikova VI, Budyak AE. Structurally and superficially bound gold in pyrite from deposits of different genetic types. Russian Geology and Geophysics. 2014;55(2):273–89. DOI:10.1016/j.rgg.2014.01.011 8. Tomashpolskiy YuYa. Poverkhnostnaya avtosegregatsiya v khimicheskikh soedineni- yakh [Superficial auto-segregation in chemical compounds]. Moscow: Nauchnyy Mir, 2013. 208 p. Russian. 9. Pervov VS, Makhonina EV, Dobrokhotova ZhV, Dubasova VS, Zarvazhnov AYu. Supramolecular model of eutectics: Functional materials based on nonautonomous phases. Inorg Mater. 2009;45(12):1478–83. DOI:10.1134/S0020168509120140 10. Gusarov VV, Malkov AA, Malygin AA, Suvorov SA. Termicheski stimulirovannye transformatsii 2-mernykh neavtonomnykh faz i uplotnenie oksidnykh polikristallich- eskikh materialov. Neorganicheskie materialy [Russ J Inorg Chem]. 1995;31(3):346–50. Russian. 11. Gusarov VV, Suvorov SA. Temperatura plavleniya lokal’no-ravnovesnykh poverkh- nostnykh faz v polikristallicheskikh sistemakh na osnove odnoy ob’emnoy fazy [Melting temperatures of locally equilibrated surficial phases in polycrystallic sys- tems based on the single phase]. Zhurnal prikladnoy khimii [Russ J Applied Chem]. 1990;63(8):1689–94. Russian. 12. Gusarov VV, Suvorov SA. Tolschina 2-mernykh neavtonomnykh faz v lokal’no- ravnovesnykh polikristallicheskikh sistemakh na osnove odnoy ob’emnoy fazy [Thickness of 2D non-autonomous phases in locally equilibrated polycrystallic sys- tems based on the single phase]. Zhurnal prikladnoy khimii [Russ J Applied Chem]. 1993;66(7):1529–34. Russian. 13. Gusarov VV. The thermal effect of melting in polycrystalline systems. Thermochim Acta. 1995;256(2):467–72. DOI:10.1016/0040-6031(94)01993-Q 14. Gusarov VV. Statika i dinamika polikristallicheskikh system na osnove tugoplavkikh oksidov [dissertation]. [St. Petersburg]: 1996. Russian. 15. Uvarov NF, Boldyrev VV. Size effects in chemistry of heterogeneous systems. Russ Chem Rev. 2001;70(4):307–29. DOI:10.1070/RC2001v070n04ABEH000638 16. Pestereva NN, Neiman AYa. Reversibility of electrosurface transfer through eutectic interfaces of MeWO4|WO3 (Me − Ca, Sr, Ba). Russ J Electrochem. 2012;48(11):1070–8. DOI:10.1134/S1023193512110134