Synthesis, Structure and Thermophysical Properties of Phosphates MNi0.5Zr1.5(PO4)3 (M = Mg, Ca, Sr)

New phosphates MNi0.5Zr1.5(PO4)3 (M = Mg, Ca, Sr) were prepared by the precipitating method. Phosphates were characterized using X-ray powder diffraction, IR-spectroscopy and electron microprobe analyses. The crystal structure of phosphates was refined by the Rietveld method. Phosphates CaNi0.5Zr1.5(PO4)3 and SrNi0.5Zr1.5(PO4)3 are shown to have been crystallized in the NaZr2(PO4)3-type structure and the phosphate MgNi0.5Zr1.5(PO4)3 was obtained as a single-phase with Sc2(WO4)3-type structure. Heat capacity of phosphate CaNi0.5Zr1.5(PO4)3 was measured in the range 7 – 650 K and increased monotonically over the entire temperature range studied. Thermal expansion of phosphate CaNi0.5Zr1.5(PO4)3 was studied in the interval 295-1073 K by the high temperature X-ray diffraction. This phosphate is similar to the best low-expansion ceramics, such as zircon, cordierite and silica glass in thermal expansion behavior.


Introduction
Framework phosphates with NaZr 2 (PO 4 ) 3 (NZP) and Sc 2 (WO 4 ) 3 (SW) type structures due to their structural peculiarities have high useful properties among them there is thermal stability, low thermal expansion behavior and thermal conduction etc. A wide variety of iso-and heterovalent chemical substitutions at all crystallographic positions of these structures allows to prepare a large number of closely related compounds and to select NZP-and SW-compositions to create new materials with desired properties, including thermophysical (thermal expansion, heat capacity, thermal conduction). There are great possibilities of preparing new compounds and solid solutions (e. g., M 2 5 . and M 4+ ), whose thermophysical properties can be predicted on crystal-chemical principles [1,2].
Compounds of the NZP family may be represented by the general crystal chemical formulae (M1) 0 1 (M2) 0 3 {[L 2 (TO 4 ) 3 ] p-} 3 with the structure containing a three-dimensionalnetwork of corner-sharing LO 6 octahedra and TO 4 tetrahedra ( Figure 1). Structural units consisting of two octahedra and three tetrahedra [L 2 (TO 4 ) 3 ] pare connected to form ribbons parallel to the c axis of the unit cell. These ribbons are linked together perpendicular to the c axis by TO 4 tetrahedra to build the three-dimensional framework. Two kinds of cavities within this framework of ribbons are formed: the first cavity, which is a strongly distorted octahedral site, M1, is located between [L 2 (TO 4 ) 3 ] punits of the ribbons. The othe r site , M 2 , i s l o c a t e d b e t w e e n t h e r i b b o n s . I n t h e prototypical NZP structure, NaZr 2 (PO 4 ) 3 , the M1 site is occupied by sodium ions Na + , the M2 site remains vacant, the L site is occupied by Zr 4+ and the T site is occupied by P 5+ . Four crystallographic sites with different coordination numbers allow substitutions by a variety of cations. Two L positions can be populated eithe r with the same three-or the same tetravalent cations. The number of individual phases considerably increases due to The valences of cations can be the same (either +3 or +4) or different (for example, +1 and +4, +2 and +3, +2 and +4, +3 and +4, +4 and +5).
T h e r e p l a c i n g a t o m s i n t h e c e n t e r o f t h e L O 6 octahedra and the TO 4 tetrahedra (a tetrahedral site normally occupied by P 5+ ) cause change s in the negative charge of the framework. This charge is compensated by the substitutions in positions M1 and M2. These positions can be filled with cations with valences ranging from +1 to +4. NZP and SW structure types are closely related: they can be described as built up from the same type of [L 2 (TO 4 ) 3 ] punits [3]. They differ from each other by the packing of these groupings ( Figure 1). As a result, holes formed in frameworks structure differ in shape and size.
Herein, synthesis, crystal structure and thermophysical data for the new complex MNi 0.5 Zr 1.5 (PO 4 ) 3 (M = Mg, Ca, Sr) are reported. The crystal chemical analysis gives us a possibility to propose that the framework phosphates CaNi 0.5 Zr 1.5 (PO 4 ) 3 a n d S r N i 0.5 Zr 1.5 (PO 4 ) 3 with NaZr 2 (PO 4 ) 3 -type structure. In these compounds the larger Ca and Sr atoms occupy the framework c a v i t i e s a n d s m a l l e r N i -a t o m s -f r a m e w o r k positions (together with zirconium). In the phosphate MgNi 0.5 Zr 1.5 (PO 4 ) 3 with SW-structure Mg-atoms occupy the framework cavities with tetrahedral coordination.

Preparation
New phosphates MNi 0.5 Zr 1.5 (PO 4 ) 3 ( M = M g , Ca, Sr) were prepared by the precipitating method starting from the following reagents: Mg(NO 3

Techniques
Phase purity of the phosphates MNi 0.5 Zr 1.5 (PO 4 ) 3 ( M = M g , C a , S r ) w a s established by powder X-ray diffraction (XRD) on a Shimadzu XRD-6000 type diffractometer using monochromatic CuK radiation. Data were recorded from 10 to 50 2 at 1°·min -1 . The data allowed indexing and assessment of phase purity. Unit cell parameters were derived from least squares refinement of the powder diffraction data from single phases.
XRD data for the structure refinement of the CaNi 0.5 Zr 1.5 (PO 4 ) 3 phosphate as collected in the angular range 8-117° 2 with a step size of 0.01° and exposure times of 4800 s. The structures were refined with the WYRIET (version 3.3) software package [4]. IR spectra were obtained by a Shimadzu FTIR-8400S spectrophotometer in the frequency range 1400-400 cm -1 . The samples were prepared by finely dispersing powder material on a ZnSe carier.
The heat capacity of the CaNi 0.5 Zr 1.5 (PO 4 ) 3 in the range from T=7 K to T=350 K was measured with a BCT-3 low-temperature adiabatic vacuum calorimeter. Its design and operational procedure were similar to those in reference [5]. An automated dynamic calorimeter (ADCTTB) operating by the principle of triple thermal bridge was used to measure the heat capacity in the range from T=330 K to T=665 K. The apparatus design and operational procedure were reported elsewhere [6]. From the calibration and testing results of the calorimeters it was established that the apparatus and the measurement procedure allowed us to obtain the 0 m , p C values of the substances in the crystalline state with an uncertainty of not more than 2 per cent from T=7 K to T=10 K, 0.5 per cent between T=10 and T=40 K, and within 0.2 per cent in the range from T=40 K to T=350 K and 1.5 per cent above 350 K.

Results and discussion
Powder XRD data of the thermally treated phosphates at the 1123 K (Fig. 2) showed that single-phase CaNi 0.5 Zr 1.5 (PO 4 ) 3 and SrNi 0.5 Zr 1.5 (PO 4 ) 3 were formed, which are isostructural to NaZr 2 (PO 4 ) 3 . These phases are rhombohedral and described by hexagonal cell parameters. The phosphate MgNi 0.5 Zr 1.5 (PO 4 ) 3 was obtained as a single-phase sample as isostructural to Sc 2 (WO 4 ) 3   Results of structural study of the CaNi 0.5 Zr 1.5 (PO 4 ) 3 phosphate have confirmed its frame structure and belonging to NZP-family.
Octahedral atoms of zirconium and nickel, and phosphorus atoms tetrahedral participate the in formation anions a skeleton.   Table 1 Coordinates, thermal displacement parameters and site occupation (q) for the basic atoms in the structure of the CaNi 0 The experimental values of the molar heat capacity of CaNi 0.5 Zr 1.5 (PO 4 ) 3 between T = 7 K and T = 650 K are presented in figure 6. It is seen that temperature dependence of heat capacity has no any special features: it gradually increases with rising temperature in the above temperature range. These functions of the investigated phosphate are given in Table 2.

Table 2
Thermodynamic functions of the crystalline CaNi 0.5 Zr 1.5 (PO 4   The temperature dependences of the parameters of the rhombohedral unit cell in the structure of the CaNi 0.5 Zr 1.5 (PO 4 ) 3 phosphate (Figure 7) were used to calculate the components of the thermal strain tensor [7]. The termal expansion coefficient of CaNi 0.5 Zr 1.5 (PO 4 ) 3 phosphate are: a = 0.5·10 -6 , c = 3.6·10 -6 , av = 1.5·10 -6 K -1 (for the 295 K). This phosphate is similar to the best low-expansion ceramics, such as zircon, cordierite and silica glass in thermal expansion behavior

Conclusion
From the results presented the following conclusions might be drawn: • New framework phosphates CaNi 0.5 Zr 1.5 (PO 4 ) 3 , SrNi 0.5 Zr 1.5 (PO 4 ) 3 and MgNi 0.5 Zr 1.5 (PO 4 ) 3 have been prepared and studied by X-ray powder diffraction, IR-spectroscopy and electron microprobe analyses • Crystal structure, temperature dependence of the heat capacity and thermal expansion for phosphate CaNi 0.5 Zr 1.5 (PO 4 ) 3 were studied.