Oumezzine Ma, Sales Herbet Bezerra, Selmi Ahmed, Hlil E K
Laboratory of Physical Chemistry of Materials, Faculty of Sciences of Monastir, University of Monastir 5019 Monastir Tunisia
UEMA/CCT, Universidade Federal de Campina Grande - UFCG Campina Grande PB Brazil.
RSC Adv. 2019 Aug 15;9(44):25627-25637. doi: 10.1039/c9ra03494c. eCollection 2019 Aug 13.
Bulk nanocrystalline samples of (La Pr )BaMnO (0.075 ≤ ≤ 0.30) manganites with a fixed carrier concentration are prepared by the sol-gel based Pechini method. Rietveld refinement of the X-ray diffraction patterns, shows the formation of single-phase compositions with rhombohedral symmetry. Upon Pr doping at the A-site, the unit cell volume and the B-O-B bond angles are reduced. FTIR spectra present a prominent absorption peak of the in-phase stretching mode (B mode) rising from the vibration of the Mn-O bond. Raman spectra at room temperature reveal a gradual shift toward lower frequencies in (E) phonon mode with increasing Pr concentration. The () measurements shows a clear ferromagnetic (FM)-paramagnetic (PM) phase transition with increasing temperature. An increase in resistivity and activation energy and a decrease in the metal-semiconductor transition ( ) and Curie temperatures ( ) was observed as a consequence of Pr doping. The results are discussed according to the change of A-site-disorder effect caused by the systematic variations of the A-site average ionic radius 〈 〉 and A-site-cation mismatch , resulting in the narrowing of the bandwidth and the decrease of the mobility of e electrons. The magneto-transport behavior in the whole measured temperature and a magnetic field can be described by a percolation model, which is in agreement with the limited experimental data of the samples for = 0.075, 0.15 and 0.30. The experimental results confirm that A-site substitution with Pr destroys the Mn-O-Mn bridges and weakens the double exchange (DE) interaction between the Mn (t e , = 2) and Mn (t e , = 3/2) ions. On the other hand, the Bean and Rodbell model has been successfully used to simulate the magnetization data of the samples with = 0.15 and = 0.22. The random replacement of La by Pr is shown to induce more disorder in the system, which is reflected in the increase of the fitted disorder parameter and spin value fluctuation. At a temperature close to room temperature, the maximum magnetic entropy change (Δ ) and the relative cooling power (RCP) of LaPrBaMnO are found to be, respectively, 1.34 J kg K and 71 J kg for a 1.5 T field change.
采用基于溶胶 - 凝胶的佩琴尼方法制备了具有固定载流子浓度的(La₁₋ₓPrₓ)BaMnO₃（0.075 ≤ x ≤ 0.30）锰氧化物的块状纳米晶样品。对X射线衍射图谱进行Rietveld精修，结果表明形成了具有菱面体对称性的单相组合物。在A位进行Pr掺杂时，晶胞体积和B - O - B键角减小。傅里叶变换红外光谱（FTIR）显示出由Mn - O键振动产生的同相拉伸模式（B模式）的一个突出吸收峰。室温下的拉曼光谱表明，随着Pr浓度的增加，(E)声子模式向低频逐渐移动。(M - H)测量结果表明，随着温度升高，出现明显的铁磁（FM）-顺磁（PM）相变。由于Pr掺杂，观察到电阻率和活化能增加，金属 - 半导体转变温度（Tₘₛ）和居里温度（Tₑ）降低。根据A位平均离子半径〈rₐ〉和A位阳离子失配δ的系统变化引起的A位无序效应的变化来讨论结果，这导致带宽变窄和e电子迁移率降低。在整个测量温度和磁场范围内的磁输运行为可以用一个渗流模型来描述，这与x = 0.075、0.15和0.30的样品的有限实验数据一致。实验结果证实，用Pr进行A位取代破坏了Mn - O - Mn桥，削弱了Mn（t₂g，S = 2）和Mn（t₃g，S = 3/2）离子之间的双交换（DE）相互作用。另一方面，Bean和Rodbell模型已成功用于模拟x = 0.15和x = 0.22的样品的磁化数据。结果表明，用Pr随机取代La会在系统中引入更多无序，这反映在拟合的无序参数和自旋值波动的增加上。在接近室温的温度下，对于1.5 T的磁场变化，La₀.₈₅Pr₀.₁₅BaMnO₃的最大磁熵变（ΔSₘ）和相对制冷功率（RCP）分别为1.34 J kg⁻¹ K⁻¹和71 J kg⁻¹。
