Compounds and Solid Solutions of Cobalt, Copper Phosphates Carlos E. Bamberger, * ,* Eliot D. Specht, † and Lawrence M. Anovitz * Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6119 Cobalt phosphates have been synthesized and their thermal behavior examined. The formation of compounds and solid solutions among cobalt and copper phosphates is studied by X-ray diffraction. Unlimited solubility exists between the pyrophosphates Co 2 P 2 O 7 and Cu 2 P 2 O 7 , the orthophos- phates Co 3 (PO 4 ) 2 and Cu 3 (PO 4 ) 2 , and Co 3 (PO 4 ) 2 and Cu 2 P 2 O 7 . X-ray diffraction indicates that the structures of cobalt and copper pyrophosphate solid solutions vary gradually from one end member to the other. The struc- tures of orthophosphate solid solutions vary in a more com- plex manner: the change is gradual from Co 3 (PO 4 ) 2 to Co 2 Cu(PO 4 ) 2 . At Co 1.5 Cu 1.5 (PO 4 ) 2 , a new structure (L) oc- curs. At 950°C, CoCu 2 (PO 4 ) 2 also exhibits this structure, which is similar to that of graftonite, Fe 3 (PO 4 ) 2 . CoCu 2 (PO 4 ) 2 is dimorphic, changing reversibly at 1000°C to a triclinic structure (H) related to that of Cu 3 (PO 4 ) 2 . From CoCu 2 (PO 4 ) 2 (H) to Cu 3 (PO 4 ) 2 , the structure appears to vary gradually. A new phosphate, Cu + Cu 2+ Co 3 (PO 4 ) 3 , has also been synthesized. I. Introduction O UR recent work on copper phosphates 1 shows that the chemistry of crystalline phosphates can be quite complex. Because the ionic radii of Cu 2+ and Co 2+ are very close (e.g., 0.057 and 0.058 nm, respectively, for coordination number IV) 2 while their electronic configurations differ, the formation and behavior of cobalt phosphates is examined, as is the formation of solid solutions between cobalt phosphates and copper phosphates. There are only a small number of cobalt phosphates known, namely metaphosphate Co(PO 3 ) 2 , 3 pyro- phosphate Co 2 P 2 O 7 , 4 and orthophosphate Co 3 (PO 4 ) 2 ; all are monoclinic. 5 The orthophosphate has a low-temperature poly- morph that forms at ∼680°C and transforms at 870°C into its stable monoclinic form. 6 A high-pressure form of Co 3 (PO 4 ) 2 is also known. 7 In general, these cobalt phosphates are analogous to copper phosphates, but no cobalt compounds similar to Cu 4 P 2 O 9 and Cu 5 P 2 O 10 have been reported. In the present study, cobalt phosphates were prepared by reaction of cobalt oxide (Co 3 O 4 ) with BPO 4 and the thermal stability of the pyrophosphates and orthophosphates was ex- amined. The formation of solid solutions between Co 2 P 2 O 7 and Cu 2 P 2 O 7 , Co 3 (PO 4 ) 2 and Cu 3 (PO 4 ) 2 , and Co 3 (PO 4 ) 2 and Cu 2 P 2 O 7 was demonstrated by X-ray diffraction (XRD), and the structure of CoCu 2 (PO 4 ) 2 was determined. Unsuccessful attempts were made to synthesize Co 4 P 2 O 9 and Co 5 P 2 O 10 and to see if CoO can be incorporated into Cu 3 (PO 4 ) 2 , and if CuO can be reciprocally incorporated into Co 3 (PO 4 ) 2 . Attempts to synthesize Cu + Co(PO 4 ), surprisingly, produced Cu + Cu 2 Co 3 (PO 4 ) 2 , a compound similar to Cu + Cu 2+ Mg 3 - (PO 4 ) 3 . 8 II. Experimental Procedure The following reagents were used: Co 3 O 4 of unknown pedi- gree was determined to be pure by XRD and free of other metals of the first transition series by inductively coupled plasma (ICP) spectrometry. Pure Cu 3 (PO 4 ) 2 and Cu 2 P 2 O 7 were prepared as described in Ref. 1 and analyzed by XRD. Cu 2 O used was ∼99% pure (General Chemical Div., New York). BPO 4 (Johnson-Matthey Electronics, Ward Hill, MA) was cal- cined at 1000°C overnight and analyzed by XRD. Cu 0 powder was purified by heating at 520°C in a stream of argon–4% hydrogen for 1 h. The compounds and mixtures were contained in crucibles of platinum or boats of fused SiO 2 or copper and were heated for 1–100 h, typically for 16 h, with intermittent homogenization by grinding. All reactions involving Cu 0 or Cu 2 O were performed under flowing argon, while all other reactions were performed in air. In most cases, the contents were not easily separated from the SiO 2 after reaction. Thus, the portions of SiO 2 attached to the reaction products were ground together with them. XRD analysis showed the initially amorphous SiO 2 crystallized as either cristobalite or quartz. When removal of unreacted Cu 0 and/or Cu 2 O was desired, powdered mixtures were reacted at room temperature usually for 15 min with an aqueous solution of 1M in FeCl 3 and 1M in HCl as used in Ref. 8. XRD analysis was performed on powders thinly coated on zero-background silicon substrate plates. It was noticed that some preparations, especially solid solutions of orthophos- phates, had a strong tendency to exhibit preferred orientation. To avoid this, these powders were ground finer and sieved through a 325 mesh screen. XRD data were collected using CuK  radiation with a Bragg–Brentano diffractometer equipped with a theta-compensating incident beam divergence slit and a graphite (002) diffracted beam monochromator. Pat- tern-processing software was used to strip K 2 and locate peaks, and lattice parameters were determined by least-squares fitting (JADE, Materials Data, Inc., Livermore, CA, 1994) with silicon powder used as an internal standard (a 0  0.54306 nm). An ICP atomic emission spectrometer (Model Iris/OES, Thermo Jarrel Ash, Franklin, MA) with an argon plasma ex- citation source was used to determine the stoichiometry of some compounds previously dissolved in warm 0.15M HNO 3 . The following wavelengths (in nanometers), calibrated from an internal mercury lamp, were used to identify the indicated el- ement: Co, 228.6; Cu, 327.3; Mg, 285.2; and P, 178.3. III. Results and Discussion (1) Syntheses of Co 2 P 2 O 7 and Co 3 (PO 4 ) 2 , and Some of Their Reactions The syntheses of Co 2 P 2 O 7 and Co 3 (PO 4 ) 2 were effected at 950°–1000°C as: 2 3 Co 3 O 4 + 2BPO 4 → Co 2 P 2 O 7 +B 2 O 3 + 1 3 O 2 (1) Co 3 O 4 + 2BPO 4 → Co 3 (PO 4 ) 2 +B 2 O 3 + 1 2 O 2 (2) Co 2 P 2 O 7 + 1 3 Co 3 O 4 → Co 3 (PO 4 ) 2 + 1 6 O 2 (3) R. L. Snyder—contributing editor Manuscript No. 190808. Received July 30, 1997; approved February 2, 1998. Supported by the Division of Materials Science, Office of Basic Energy Sciences, U.S. Department of Energy at Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corp. under Contract No. DE-AC05-96OR22464. * Member, American Ceramic Society. * Chemical and Analytical Sciences Division. ² Metals and Ceramics Division. J. Am. Ceram. Soc., 81 [11] 2799–804 (1998) J ournal 2799