Istraživanje lokalne strukture kompleksnih oksida titana i tantala Larmorovom precesijom neutrona

master's thesis

• Title (english): Investigation of local structure in complex oxides of titanium and tantalum using neutron Larmor precession
• Author: Luka Rogić
• Mentor: Damjan Pelc (mentor)
• Committee member: Damjan Pelc (predsjednik povjerenstva)
• Committee member: Ivan Kupčić (član povjerenstva)
• Committee member: Damir Pajić (član povjerenstva)
• Committee member: Zoran Rukelj (član povjerenstva)
• Committee member: Mihael Srđan Grbić (član povjerenstva)
• Granter: University of Zagreb; Faculty of Science; (Department of Physics); Zagreb
• Defense date and country: 2024-07-18, Croatia
• Scientific / art field, discipline and subdiscipline: NATURAL SCIENCES; Physics

Abstract

U ovom radu predstavljeni su rezultati Larmorove difrakcije neutrona na dva glavna predstavnika kvantnih paraelektrika, stroncijevom titanatu i kalijevom tantalatu. Istraženi su nedopirani uzorci, kao i uzorci dopirani niobijem i vakancijama kisika. Promatranjem promjene razmaka rešetke utvrđeno je da negativna termalna ekspanzija nedopiranog stroncijevog titanata nestaje pri dopiranju. Ovaj rezultat implicira snažan utjecaj dopiranja na anharmoničnost fonona u stroncijevom titanatu. Isti rezultat nije pronađen u kalijevom tantalatu, gdje je termalna ekspanzija pozitivna i u nedopiranom i dopiranom uzorku. Također, određene su i distribucije razmaka rešetke stroncijevog titanata, u kojima je uočeno cijepanje na dvije srednje vrijednosti razmaka rešetke, što je u skladu s poznatim antiferodistortivnim faznim prijelazom. Iz distribucija je moguće odrediti međusobni omjer zastupljenosti vrijednosti razmaka u uzorku, kao i širinu distribucije koja daje ideju o unutarnjoj deformaciji uzoraka. Dobivena vrijednost unutarnje deformacije u nedopiranom stroncijevom titanatu je veća od kritične vrijednosti elastične deformacije koja inducira feroelektričnost, što može objasniti formaciju polarnih nanoregija pri niskim temperaturama.

Abstract (english)

representative quantum paraelectrics: strontium titanate and potassium tantalate. Both undoped samples and samples doped with niobium and oxygen vacancies were investigated. By looking into the change of lattice-spacings, we discover that the negative thermal expansion of strontium titanate vanishes with very low doping. This result implies that doping has a strong effect on phonon anharmonicity in strontium titanate. The same result is not found in potassium tantalate, where thermal expansion is positive in both the undoped and doped samples. Additionally, distributions of lattice-spacings of strontium titanate are determined for various temperatures and a splitting of distributions into two mean values is observed. This is in agreement with the known antiferrodistortive phase transition. From these distributions, it is possible to determine the ratio of the mean value populations, as well as the width of the distribution which provides an idea of intrinsic strain. The obtained intrinsic strain values in undoped strontium titanate are above the critical value of elastic strain that induces ferroelectricity, which can explain the formation of polar nanoregions at low temperatures.

• Keywords: stroncijev titanat; kalijev tantalat; Larmorova difrakcija; kvantni paraelektrik; polarne nanoregije; fononska anharmoničnost
• Keywords (english): strontium titanate; potassium tantalate; Larmor diffraction; quantum paraelectric; polar nanoregions; phonon anharmonicity
• Language: croatian
• URN:NBN: urn:nbn:hr:217:489906
• Project: Number: UIP-2020-02-9494; Title: Kolektivna pobuđenja i lokalna struktura u nekonvencionalnim supravodičima; Title: Collective excitations and local structure of unconventional superconductors; Acronym: CELUS; Leader: Damjan Pelc; Jurisdiction: Croatia; Funder: Hrvatska zaklada za znanost; Funding stream: Installation Research Projects
• Study programme: Title: The University Integrated Undergraduate and Graduate Programme in Physics; Research Physics; specializations in: Research Physics; Course: Research Physics; Study programme type: university; Study level: integrated undergraduate and graduate; Academic / professional title: sveučilišni magistar fizike (sveučilišni magistar fizike)
• Type of resource: Text
• File origin: Born digital
• Access conditions: Embargoed access; Embargo expiration date: 2025-07-18
• Created on: 2024-09-09 11:35:59