RESULTADOS Y DISCUSIÓN

The high temperature structural properties of YMnO3 were rst reported in the 1960s

with a report of a ferrielectric transition with a concurrent structural transition from P63cm at ambient temperature to the centrosymmetric space group P63/mcm at high

temperature [153]. However, the samples used were Bi-ux grown and the eect of impurities would be expected to aect the transition temperature and possibly introduce strain into the crystals.

There have been many investigations over the years into the high temperature phase diagram of YMnO3 using a variety of techniques. The results of these are tabulated in

table 6.1. From examination of the range of transition temperatures proposed in these experiments using a wide range of methods it becomes clear that there is no general consensus beyond that there is at least one high temperature transition above 900 K.

An intermediate phase between P63/mmc and P63cm has been discussed by various

authors. The rst investigation, by Lonkai et al. in 2004 [163], analysed the possible transition paths between P63/mmc and P63cm using group theoretical arguments in

Reference TC(K) TS(K) Technique

Coeuré et al. [154] 913 Dielectric permittivity

Ismailzade and Kizhaev [153] 933 Pyroelectric current

and SXD

Šukaszewicz and _{1275 SXD}

Karat-Kalici«ska [155]

Katsufuji et al. (2001) [156] 910 Resistivity

Katsufuji et al. (2002) [157] ≥1000 ≥1000 PXRD

Nénert et al. (2005) [158] 1020 1273 SXD

Nénert et al. (2007) [159] 1125 1350 Powder DTC

Jeong et al. [160] ≥1200 PND

Choi et al. [161] ≈880 Resistivity

Kim et al. [162] ≈920 PXRD (MEM)

Table 6.1: The transition temperatures previously reported in the literature for the ferrielectric (TC) and unit-cell tripling (TS) transitions of YMnO3. In the

Technique column PXRD = powder x-ray diraction, SXD = single-crystal x- ray diraction, PND = powder neutron diraction, DTC = dierential thermal

calorimetry and MEM = maximum entropy method.

conjunction with neutron diraction on the isostructural compound TmMnO3 and con-

cluded that there was no evidence for an intermediate phase. Further studies by Nénert et al. [158, 159], using synchrotron x-ray diraction and dierential thermal calorimetry (DTC), and a meta-analysis by Abrahams [164] were in favour of a P63/mcm inter-

mediate phase. Clarifying the details of the transition is crucial for understanding the mechanism of the ferrielectricity and therefore also central to the study of the magneto- electric coupling in the multiferroic state of YMnO3.

The determination of the nature and temperature of the transition(s) is hampered by the fact that many physical property measurements are dicult at these very high tem- peratures. In particular the relatively small charge-transfer gap causes signicant leakage currents in such measurements as dielectric permittivity. TC would also be expected to

be sensitive to impurities.

The high temperature phase of YMnO3 above 1300 K as mentioned has been generally

agreed to be theP63/mmcstructure which is the undistorted aristotype for the ambient

temperature P63cm structure. The transition from the high temperature aristotype

P63/mmc structure to theP63cm structure involves tilting of the trigonal bipyramids,

corrugation of the Y3+ _{ion layer leading to tripling of the unit cell and loss of mirror}

unequal between the two crystallographically inequivalent Y sites in theP63cmstructure.

If the assumption of a second order transition between the high temperature P63/mmc

and ambient temperatureP63cmphases is used (there are no indications from previous

studies of physical property discontinuities that would require a rst-order transition) there are multiple possible transition paths allowed by Landau theory. The possible sequences of transitions, based upon the assumption of the transition(s) being second order in nature, are illustrated in gure 6.4. The K1 mode causes tripling of the unit

cell by allowing displacement of the O-Mn-O axis in the ab-plane and leads to the space group P63/mcm. Γ−2 consists of polar displacements in the c-axis direction and leads

to space group P63mc. The K3 mode leading to the space group P63cm consists of

antiferrodistortive displacements giving corrugation of the Y-ion plane, tilts of the MnO5

trigonal bipyramids and unit cell tripling. The transitions all involve a unit-cell tripling K mode but the order of this symmetry breaking and any polar displacement varies. The transition, via K modes, between the high temperature P63/mmc structure and

either P63cm or P63/mcm will be referred to as the unit cell tripling transition with

transition temperatureTS. The ferrielectric Curie temperature,TC, may be the same as

or dierent fromTS depending on the transition path taken.

Due to the dierences in the order of the distortion modes acquiring non-zero ampli- tudes between the dierent possible transition paths, the best initial test of the transition path is to identify the unit cell tripling temperature,TSand any correspondence between

this and the corrugation of the Y3+ _{ion layer and tilting of the Mn trigonal bipyramids.}

The new avenue taken in this thesis compared to previous work is that the experi- ments described in this chapter are high-resolution neutron diraction experiments on polycrystalline samples. Neutron diraction has a much greater sensitivity to the oxy- gen positions than x-ray diraction and the combination of neutrons and the use of

P63cm P63mc P63/mmc K3 Γ−₂ P63/mcm K1 Γ−₂ K1

Figure 6.4: The possible second order phase transition paths for YMnO3 be-

tween the high temperature centrosymmetric phase and the ambient tempera- ture polar phase.

HRPD at ISIS, the highest resolution beamline of its type in the world, over the entire temperature region of interest allows us to perform the most thorough study to date of the high temperature phase transitions in YMnO3. Furthermore, careful synthesis of

the polycrystalline samples ensured clean samples with a minimal impurity content and minimised potential problems caused by strain.