JOURNAL OF SOLID STATE CHEMISTRY 137, 19 — 27 (1998) ARTICLE NO. SC977653 Synthesis, Structure, and Properties of Sodium or Potassium-Doped Lanthanum Orthomanganites from NaCl or KCl Flux 1 Ram Niwas Singh,* C. Shivakumara,* N. Y. Vasanthacharya,* S. Subramanian,- M. S. Hegde,* H. Rajagopal,‡ and A. Sequeira‡ * Solid State and Structural Chemistry Unit, and - Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India; and ‡ Solid State Physics Division, Bhabha Atomic Research Center, Trombay, Mumbai 400 085, India Received February 2, 1997; in revised form October 6, 1997; accepted October 9, 1997 Lanthanum and oxygen-deficient La 1xy A x MnO 3 ( A  Na or K) ferromagnetic oxide phases have been synthesized from NaCl or KCl flux, starting from La 2 O 3 and MnCO 3 at 900°C. Composition of the final product could be varied by varying initial composition La y MnO 3 (0.7(y(1). Final composition of these oxides was obtained from chemical analysis of elements present. As the lanthanum content increased, Na or K content decreased and the structure changed from rhombohedral to or- thorhombic symmetry. Mn 4 concentration was 27% in the sodium doped manganite, La 0.82 Na 0.13 MnO 2.93 with Curie tem- perature 325 K, and Mn 4 was 25% in the potassium doped La 0.82 K 0.08 MnO 2.89 with T c of 310 K. Structure of these two phases have been refined from neutron diffraction studies. Stoichiomet- ric La 1x A x MnO 3 (A  Na or K) have been obtained for the initial composition La y MnO 3 ( y  0.85). Orthorhombic phases obtained were found to be Mn-deficient La 1x Na x Mn 1x Mn 1y O 3 (xK0.02; yK0.05), which were ferromagnetic insulators, even though total Mn 4 contents were as high as 29%.  1998 Academic Press 1. INTRODUCTION Lanthanum orthomanganites of general formula La  A  MnO  (A"Ca, Sr, Ba, and Pb) have been known for a long time (1, 2). Recent observation of colossal mag- netoresistance (CMR"(R  !R  )/R  ) in La  A  MnO  has generated new interest in these materials (3—6). Exact reason for CMR is a subject of current interest (7, 8). It is desirable to have CMR at 300 K for commercial applica- tion. Highest CMR is observed near the ferromagnetic transition temperature ¹  , and hence the search for new materials having ¹  near 300 K. Goodenough (9) has in- dicated that La  MnO  ( y(1) showed ¹  165—220 K. Re- cently we had shown that a ¹  of 220 K is obtained in the bulk solid as well as thin films of lanthanum-deficient oxide  Contribution 1275 from Solid State and Structural Chemistry Unit. having nominal composition La  MnO  (10). Further, films of such material showed over 80% CMR at 220 K at 6.5 ¹. Oxygen ion deficiency in La  Ba  MnO  and La  Pb  MnO  showed a decrease in ¹  , but there is an increase in CMR(11, 12). Cation as well as anion deficiency is likely to be one of the reasons for such a large CMR in La  MnO  (y(1). Solid state preparation of La  MnO  invariably gave multiphasic mixtures with Mn  O  as an impurity. In an attempt to eliminate the Mn  O  impurity via water soluble manganese salts, synthesis of lanthanum- deficient phases were attempted using neutral NaCl and KCl fluxes. Since Na or K can substitute for La (13), we obtained ferromagnetic Na or K doped lanthanum manga- nites. Among the large number of Na or K doped lan- thanum manganites synthesized by this method, epitaxial thin film of La  Na  MnO  was made by pulsed laser deposition, and the thin film showed over 70% CMR at 290 K in 6T magnetic field (14). Composition of these phases has been derived by chemical analysis of La, Na/K, Mn, and oxygen independently. Here we report a new method to synthesize Na or K doped lanthanum orthomanganites by neutral NaCl or KCl flux. Structure, electrical, and mag- netic properties of these oxides have been presented. 2. EXPERIMENTAL La  O  and MnCO  for a starting composition La  MnO  (0.6(y(1) were mixed with dry NaCl in the weight ratio 1 : 16. The mixture was heated to 900°C for 24h in an alumina crucible. The melt was furnace cooled and washed in hot, distilled water until no Na or Cl was detected in the filtrate. The black solid was filtered and dried at 110°C. Similarly, a series of oxides were prepared in KCl melt. Powder X-ray diffraction (JEOL JDX-8P) pat- terns were recorded at a scan rate of 2°/min with CuK (1.5418 A s ). Final composition of these oxides was obtained from chemical analysis of the elements present. Neutron diffraction patterns for the two samples with starting 19 0022-4596/98 $25.00 Copyright  1998 by Academic Press All rights of reproduction in any form reserved.