X-ray powder diffraction data for Ba 2 Cu 3 Pr0 696±001 V. E. Lamberti, M. A. Rodriguez, and A. Navrotsky Princeton Materials Institute and Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544 (Received 12 October 1994; accepted for publication 25 February 1995) The X-ray powder diffraction pattern for the title compound is reported in the range 5=£20«125°. The sample was prepared through solid-state reaction of BaCO 3 , CuO, and Pr 6 O n , and characterized with respect to oxygen content through iodometric titration. Refined parameters for the orthorhombic (space group Pmmm) unit cell are a = 3.8587(2) A; b = 3.9302(1) A; c = 11.7126(3) A; alb = 0.98181(6); ale = 0.32945(2); blc = 0.33555(1); Z = 1; D x = 6.705(2) Mgm" 3 ; F 30 = 48(0.005,127). V = 177.62(1) A j ; formula wt. = 717.48(16) g mol" 1 ; SS/FOM: I. INTRODUCTION The nonstoichiometric quaternary oxide Ba 2 Cu 3 Pr0 7 _^ (Prl23) is characterized by a variety of curious features, in- cluding the singular absence of superconductivity among all the rare-earth analogues of Ba 2 Cu 3 YO 7 _^ (Soderholm et ah, 1987; Dalichaouch et al, 1988), an unusually high Neel tem- perature (17 K) for ordering of Pr moments (Li et al., 1989), and a heavy-fermion-like specific heat (Phillips et ah, 1991). The implications of these properties for the mechanism of high-T c superconductivity, as well as for the design of prac- tical devices such as nearly-lattice-matched S-I-S junctions, have made Prl23 the subject of a great number of theoretical and experimental investigations (Radousky, 1992). Perhaps of greatest controversy is the charge distribution within the unit cell, especially in connection with the formal ionic va- lence (3 + , intermediate, or 4+) of the Pr ion {ibid.). Single-crystal structural studies (Lowe-Ma and Van- derah, 1992) and Rietveld refinements of X-ray (Kinoshita et al, 1988) and neutron powder diffraction data (Ganguli et al, 1989; Lopez-Morales et al, 1990; Neumeier et al, 1990; Guillaume et al, 1993; Malik et al, 1994) confirm that, for sufficiently small values of S, Ba 2 Cu 3 PrO 7 _ (5 is iso- structural in space group Pmmm with its rare-earth and yt- trium congeners. However, crystal-chemical trends across the 123 series are often broken at Prl23 by structural anoma- lies such as a rare-earth polyhedral volume that is signifi- cantly less than that of Ndl23 (Lowe-Ma and Vanderah, 1992) or, in the notation of Radousky (1992), subtle contrac- tions in the Cu(2)-O(4) (Kinoshita et al., 1988; Lowe-Ma and Vanderah, 1992; Booth et al, 1994) and Pr-O(2) (Guil- laume et al, 1993) bond lengths. In addition, XRD/DTA data suggestive of limited disordered cation mixing across the Ba and Pr sites have been obtained (McCallum and Park, 1994). In this paper we report X-ray powder diffraction data for the orthorhombic phase Ba 2 Cu 3 PrO 6 96±0 01 suitable for inclusion in the powder diffraction file. II. SAMPLE PREPARATION Polycrystalline Ba 2 Cu 3 PrO 7 _ (5 was prepared by conven- tional solid-state methods from ^sar/Johnson Matthey BaCO 3 (99.99%), CuO (99.999%), and Pr 6 O n (99.99%). The starting materials, once dried for 72 h at either 473 (carbon- ate) or 1073 K (oxides), were grouped stoichiometrically in an agate mortar, ground to a visibly homogeneous mixture, and pelleted into half-gram discs of diameter 0.5 in. The pellets were prefired in air for 15 h at 1123 K and then calcined for ca. 12 d at 1213 K. Cycles of grinding and pelletization were performed after the prefiring and at the end of each three-day interval of the calcination. The calci- nation product was annealed for 48 h at 673 K in flowing oxygen and then slowly cooled (—10° min" 1 ) to room tem- perature. Polycrystalline magnesia crucibles were used in all processing steps. Cursory diffraction patterns were taken at various intermediate points in the synthesis to monitor the presence of starting materials and common impurity phases such as BaCuPr 2 O 5 and BaCuO 2 . The oxygen content ( 7 - <5) of the black final product was determined by iodometric titration against KlO 3 -standardized Na 2 S 2 O 3 (Nazzal et al, 1988; Zakharchuk et al, 1991; Zhou and Navrotsky, 1992); the error represents the 95% confi- dence level. It should be emphasized that this error reflects only the propagated statistical uncertainties of the titrations, and excludes systematic errors arising from, for example, uncertainties in the assumed cation stoichiometry or the con- centration of titrant. The product was stored over CaSO 4 des- iccant until needed, as at least Ba 2 Cu 3 YO 7 _ 5 is known to be thermodynamically unstable with respect to corrosion by wa- ter and carbon dioxide at room temperature (Zhou and Navrotsky, 1993). III. POWDER DATA COLLECTION AND ANALYSIS Powder data were collected on a Scintag PAD V diffrac- tometer controlled by a DEC Micro VAX 3100 computer. In- strumental and scan parameters are summarized in Table I. The powder sample, having been first ground in an agate mortar to an average particle size of about 10 /xm and then mixed with standard silicon (NIST SRM #640b), was mounted in a deep-well specimen holder. The quantity of internal standard was adjusted so that the intensity of its (111) reflection roughly equaled that of the (013) reflection of Prl23. Three complete scans were performed, with the sample holder rotated successively by 120° after each of the first two scans. Analysis of the powder data was conducted entirely with algorithms resident in the Scintag DMS (diffraction manage- ment system) software. Discrete Bragg reflections were lo- cated with the PEAKFINDER program, which employs a second-derivative test. Groups of overlapped peaks were de- 207 Powder Diffraction 10 (3), September 1995 0885-7156/95/10(3)/207/3/$6.00 M995 JCPDS-ICDD 207