Hydrothermal Synthesis And Characterization Of The Undoped Rare Earth Manganites

In the past few decades, the research development of rare earth manganites hasalways been an interesting subject due to this family has diverse structures and richoptic, electric and magnetic properties. Generally, this family can fall into twodistinct groups according to the radiuses of rare earth cations: R'-Mn-O (R'=Y, Ho,Er, Tm, Yb, Lu) and R''-Mn-O (R''=La, Pr, Nd, Sm-Dy). Both of the two groupscontain two chemical compounds, RMnO3 and RMn2O5, but differ in the crystalstructures of RMnO3: hexagonal in the R'group and orthorhombic perovskite-like inthe R''group. The latter rare earth manganites R''MnO3 (R''=La, Pr, Nd, Sm-Dy)doped with calcium, strontium and barium exhibit a sizeable magnetoresistive effect,which draws much attention ever since it was discovered.Rare earth manganites are traditionally synthesized by solid state reaction orcitrate precursor method. Though high temperature solid state reaction has theadvantages of simple reaction and easy operation, it requires relatively hightemperature (above 1000℃) and in some cases may lead to the formation ofundesirable phases. In contrast, in the hydrothermal system the boiling point of waterincreases and great pressure is generated, both of which improve the solubility ofagents. Thus the reaction can be carried out in a homogeneous system at much lowertemperature. And the low temperature of hydrothermal method also favors theformation of some metastable phases. The differences between hydrothermal methodand solid state reaction lie in the fact that the key step of the latter reaction is theatom diffusion and the key step of the former is the process of dissolution andre-crystallization which favors the preparation of inorganic compounds with highpurity and perfect crystallization.In our work, we have hydrothermally synthesized eight undoped rare earthmanganites by searching for suitable reaction conditions, such as the mol ratio ofagents, alkalinity, concentration and temperature etc. The hydrothermal products arecompared with those obtained by solid state reaction or citrate method for thepurpose to establish the relationship between synthesis methods and structures andproperties of the final products.Two stable rare earth manganites, ErMnO3 and YMnO3 with hexagonalstructures have been successfully synthesized by hydrothermal method at 250℃for48 hours. In the SEM photos we observed the products have high purity and theparticles were composed by hexagonal sheets, which were accumulated layer bylayer. The magnetic measurement of ErMnO3 showed that the paramagnetism of Er3+was relatively great and thereby masked the antiferromagnetic transition. Concerningthe magnetic property of YMnO3, it presented a spin-glass state at low temperaturedue to the competition of antiferromagnetism and ferromagnetism caused by thecoexistence of Mn3+ and Mn4+.By hydrothermal synthesis, two metastable manganites HoMnO3 and DyMnO3,and a stable compound SmMnO3 with orthorhombic structures have been prepared.Compared with the products of solid state reaction, hydrothermal samples hadregular shapes and crystallized well. The size of the particles of HoMnO3 andDyMnO3 was about 40~50μm. The magnetic susceptibility curves of HoMnO3 andDyMnO3 supplied a Curie-Weiss-like information at about 10 K, although they wereactually antiferromagnets under 40K, which was also ascribed to the largeparamagnetisms of Ho3+ and Dy3+.Another three orthorhombic RMn2O5 (R=La, Pr, Nd) compounds containingmixed-valence manganese cations were synthesized through hydrothermal method...