Strobel Sabine, Choudhury Amitava, Dorhout Peter K, Lipp Christian, Schleid Thomas
Institute for Inorganic Chemistry, Universitaet Stuttgart, Stuttgart, Germany.
Inorg Chem. 2008 Jun 2;47(11):4936-44. doi: 10.1021/ic800233c. Epub 2008 May 7.
The rare-earth metal(III) oxide selenides of the formula La4O4Se[Se2], Ce4O4Se[Se2], Pr4O4Se[Se2], Nd4O4Se[Se2], and Sm4O4Se[Se2] were synthesized from a mixture of the elements with selenium dioxide as the oxygen source at 750 degrees C. Single crystal X-ray diffraction was used to determine their crystal structures. The isostructural compounds M4O4Se[Se2] (M=La, Ce, Pr, Nd, Sm) crystallize in the orthorhombic space group Amm2 with cell dimensions a=857.94(7), b=409.44(4), c=1316.49(8) pm for M=La; a=851.37(6), b=404.82(3), c=1296.83(9) pm for M=Ce; a=849.92(6), b=402.78(3), c=1292.57(9) pm for M=Pr; a=845.68(4), b=398.83(2), c=1282.45(7) pm for M=Nd; and a=840.08(5), b=394.04(3), c=1263.83(6) pm for M=Sm (Z=2). In their crystal structures, Se2- anions as well as [Se-Se]2- dumbbells interconnect {[M4O4]4+} infinity 2 layers. These layers are composed of three crystallographically different, distorted [OM4]10+ tetrahedra, which are linked via four common edges. The compounds exhibit strong Raman active modes at around 215 cm(-1), which can be assigned to the Se-Se stretching vibration. Optical band gaps for La4O4Se[Se2], Ce4O4Se[Se2], Pr4O4Se[Se2], Nd4O4Se[Se2], and Sm4O4Se[Se2] were derived from diffuse reflectance spectra. The energy values at which absorption takes place are typical for semiconducting materials. For the compounds M4O4Se[Se2] (M=La, Pr, Nd, Sm) the fundamental band gaps, caused by transitions from the valence band to the conduction band (VB-CB), lie around 1.9 eV, while for M=Ce an absorption edge occurs at around 1.7 eV, which can be assigned to f-d transitions of Ce3+. Magnetic susceptibility measurements of Ce4O4Se[Se2] and Nd4O4Se[Se2] show Curie-Weiss behavior above 150 K with derived experimental magnetic moments of 2.5 micro B/Ce and 3.7 micro B/Nd and Weiss constants of theta p=-64.9 K and theta p=-27.8 K for the cerium and neodymium compounds, respectively. Down to 1.8 K no long-range magnetic ordering could be detected. Thus, the large negative values for theta p indicate the presence of strong magnetic frustration within the compounds, which is due to the geometric arrangement of the magnetic sublattice in form of [OM4]10+ tetrahedra.
式为La4O4Se[Se2]、Ce4O4Se[Se2]、Pr4O4Se[Se2]、Nd4O4Se[Se2]和Sm4O4Se[Se2]的稀土金属(III)氧化物硒化物是在750摄氏度下由元素混合物与二氧化硒作为氧源合成的。使用单晶X射线衍射确定它们的晶体结构。同构化合物M4O4Se[Se2]（M = La、Ce、Pr、Nd、Sm）在正交空间群Amm2中结晶，对于M = La，晶胞参数a = 857.94(7)、b = 409.44(4)、c = 1316.49(8) pm；对于M = Ce，a = 851.37(6)、b = 404.82(3)、c = 1296.83(9) pm；对于M = Pr，a = 849.92(6)、b = 402.78(3)、c = 1292.57(9) pm；对于M = Nd，a = 845.68(4)、b = 398.83(2)、c = 1282.45(7) pm；对于M = Sm，a = 840.08(5)、b = 394.04(3)、c = 1263.83(6) pm（Z = 2）。在它们的晶体结构中，Se2-阴离子以及[Se - Se]2-哑铃状结构连接{[M4O4]4+}∞2层。这些层由三个晶体学上不同的、扭曲的[OM4]10+四面体组成，它们通过四个公共边相连。这些化合物在215 cm(-1)左右表现出强烈的拉曼活性模式，可归因于Se - Se伸缩振动。La4O4Se[Se2]、Ce4O4Se[Se2]、Pr4O4Se[Se2]、Nd4O4Se[Se2]和Sm4O4Se[Se2]的光学带隙由漫反射光谱得出。吸收发生的能量值是半导体材料的典型值。对于化合物M4O4Se[Se2]（M = La、Pr、Nd、Sm），由价带向导带（VB - CB）跃迁引起的基本带隙约为1.9 eV，而对于M = Ce，吸收边出现在约1.7 eV处，可归因于Ce3+的f - d跃迁。Ce4O4Se[Se2]和Nd4O4Se[Se2]的磁化率测量表明，在150 K以上呈现居里 - 外斯行为，铈和钕化合物的实验得出的磁矩分别为2.5 μB/Ce和3.7 μB/Nd，外斯常数分别为θp = -64.9 K和θp = -27.8 K。低至1.8 K时未检测到长程磁有序。因此，θp的大负值表明化合物中存在强磁阻挫，这是由于[OM4]10+四面体形式的磁亚晶格的几何排列所致。
