Centre for Science at Extreme Conditions and School of Chemistry, University of Edinburgh, King's Buildings, Mayfield Road, Edinburgh, EH9 3JZ, United Kingdom.
J Am Chem Soc. 2010 Apr 7;132(13):4822-9. doi: 10.1021/ja910745b.
A remarkable electronic flexibility and colossal magnetoresistance effects have been discovered in the perovskite oxynitrides EuWO(1+x)N(2-x). Ammonolysis of Eu(2)W(2)O(9) yields scheelite-type intermediates EuWO(4-y)N(y) with a very small degree of nitride substitution (y = 0.04) and then EuWO(1+x)N(2-x) perovskites that show a wide range of compositions -0.16 <or= x <or= 0.46. The cubic lattice parameter varies linearly with x, but electron microscopy reveals a tetragonal superstructure. The previously unobserved x < 0 regime corresponds to oxidation of Eu (hole doping of the Eu:4f band), whereas x > 0 materials have chemical reduction of W (electron doping of the W:5d band). Hence, both the Eu and W oxidation states and the hole/electron doping are tuned by varying the O/N ratio. EuWO(1+x)N(2-x) phases order ferromagnetically at 12 K, and colossal magnetoresistances (CMR) are observed in the least doped (x = -0.04) sample. Distinct mechanisms for the hole and electron magnetotransport regimes are identified.
在钙钛矿氧氮化物 EuWO(1+x)N(2-x) 中发现了显著的电子柔韧性和巨大磁电阻效应。Eu(2)W(2)O(9) 的氨解生成白钨矿型中间体 EuWO(4-y)N(y)，其氮化物取代程度非常小(y = 0.04)，然后生成 EuWO(1+x)N(2-x) 钙钛矿，其组成范围很广(-0.16 <= x <= 0.46)。立方晶格参数随 x 线性变化，但电子显微镜显示出四方超结构。以前未观察到的 x < 0 区域对应于 Eu 的氧化(对 Eu:4f 带的空穴掺杂)，而 x > 0 材料则发生 W 的化学还原(对 W:5d 带的电子掺杂)。因此，通过改变 O/N 比可以调节 Eu 和 W 的氧化态以及空穴/电子掺杂。EuWO(1+x)N(2-x) 相在 12 K 下呈铁磁性有序，在掺杂最少的(x = -0.04)样品中观察到巨大磁电阻效应。确定了空穴和电子磁输运机制。
