鹼土族鉻氧化物介電性質之研究

Abstract

摘 要 不同價位鉻的氧化物Ca2+Cr23+O42-, Ca22+Cr23+O52-, 和A32+Cr25+O82- (A = Ca、 Sr、Ba)以及Mn參雜濃度不同Sr3Cr2-xMnxO8(x = 0.0, 0.1, 0.2)之介電性質研究。這一系列的化合物是利用固態方法於不同的鍛燒溫度(TA)、退火過程，和不同的通氣鍛燒時間等製程以控制氧化程度。我們量測它們的介電性質並觀察它們的晶體結構. 藉由使用 X-ray 粉末分析法, 我們發現此系列化合物都同擁有相似的正交晶系結構,並且從最近的文獻中得知,降溫的過程中Ba3Cr2O8在70K以及Sr3Cr2O8在285K都存在一個六角晶系到單斜晶系的Jahn - Teller結構相變。我們以溫度和頻率作變數來量測介電常數(ε’)，介電損失(tan δ)，複數modulus (M’, M’’) 和 AC電導率以及磁化率來分析這些化合物的特性。除了Ca2Cr2O5，多數樣品在室溫下，可量到巨介電常數，而且發現這和晶體的結構有強關聯性，特別是Ba3Cr2O8在Jahn – Teller效應影響下的介電常數變化。另一個Sr3Cr2-xMnxO8(x = 0.0, 0.1, 0.2)系列，隨著Mn的參雜濃度提高，改變了晶格參數。 所有的樣品在ε’ (T) 的圖中，介電常數皆呈階梯式增加，並且皆對應一個峰值在 tan δ 圖中. 我們利用複數modulus 的計算來辨別出 grain 和 grain boundary 對整個介電系統的影響, 並發現在高溫時 grain boundary 對整個系統較 grain 佔有更大的影響. 目前在此篇論文的發現Ba3Cr2O8和Sr3Cr2O8在Jahn – Teller 結構相變之下，由於Ba3Cr2O8的 TJT = 70K 較Sr3Cr2O8的TJT = 285K更為低溫，是提供Jahn – Teller效應下，提供一個雜訊更少的環境的良好印證環境。Sr3Cr2-xMnxO8(x = 0.0, 0.1, 0.2)也因為參雜濃度改變，而改變了晶格參數，改變了介電常數量級。此外，Cr3+的CaCr2O4存在巨介電現象於接近室溫時(ε’~ 104)，並且發現介電常數展現另一個極大值~ 40 K 和長程反 鐵磁有序性有關，皆為介電性質提供更大的研究潛力。

Abstract The synthesis, characterization and dielectric properties of polycrystalline alkaline earth chromates Ca2+Cr23+O42-, Ca22+Cr23+O52-, and A32+Cr25+O82- (A = Ca, Sr, and Ba) compounds were investigated. Powder X-ray diffraction patterns reveal that all the samples are single phase without any impurity. The dielectric properties of these compounds were studied in the frequency range from 20 Hz to 1 MHz between 20 K and 320 K. The Cr3+ valence contains CaCr2O4 sample exhibits a high dielectric permittivity (ε' ~ 104) at room temperature with frequency dispersion. In addition, ε'(T) also shows maxima at ~ 40 K. The corresponding tanδ (T) curves exhibit two relaxations at T < 40 K and ~ 260 K. The peak temperature TP of tanδ shifts to a higher temperature as the frequency increases in both relaxations. The overall behavior of high temperature ε' and tanδ is similar to that observed in giant dielectric constant materials. The observed giant dielectric response was explained in terms of internal (grain boundary) barrier layer capacitance (IBLC) effect and maxima at ~ 40 K is related to the development of long-range antiferromagnetic order. Moreover, calcium rich Cr3+ valence Ca2Cr2O5 sample shows very small value of ε' compare to the value of CaCr2O4. The ε'(T) curves of A3Cr2O8 (A = Ca, Sr, and Ba) samples reveal that the value of real dielectric permittivity decreases with increasing A-site ionic radius. The tanδ(T) curves of these compounds exhibit dielectric relaxation at high temperature. These peak temperatures also found to be decreased with increasing A-site ionic radius. The peak temperatures TP of tanδ shifts to a higher temperature as the frequency increases in all samples. The structural, orbital or spin fluctuations are likely origin for the observed results of Cr5+ valence compounds.