UNIWERSYTET ŚLĄSKI Instytut Fizyki im. A. Chełkowskiego

Zakład Fizyki Ferroelektryków ul. Uniwersytecka 4

40-007 Katowice

tel.: +48 32 359 1478

E-mail: Krystian.Roleder@us.edu.pl fax: +48 32 258 8431

Katowice, November 7th, 2014

Review of the thesis “Thermodynamic studies of pure and of manganese doped lead titanate

and sodium niobate with perovskite structure” by

Dietmar Kobertz MSc. from Forschungszentrum Jülich in Germany

Supervisor: dr hab. Andrzej Molak Department of Ferroelectric Physics, Institute of Physics, University of Silesia 1. General comments:

The doctoral thesis has been prepared according to the “old rule” of Polish Law on Higher Education, which allows a thesis to be presented as a separate printed document. The thesis consists of 14 chapters containing introductive information about the thesis and its aims, description of experimental methods used, detailed presentation of experimental results with conclusions following them, as well as final remarks, annex, and numerous references. All of the chapters are written very interestingly, with no redundant information, and are well elaborated. It has to be stressed that the thesis is very well organised with - not often found in nowadays theses in natural sciences - such chapters as “Symbols and Notation”, “List of Abbreviations”, “List of Tables”, “List of Figures”, and very helpful “Index”. Even from this point of view the form of this thesis is by far different (and better) from those usually presented in the field of physics. In the era of nanotechnologies and nano-ferroics physics, another unusual feature of this thesis is its subject, i.e. the thermodynamic studies of perovskites. While structural, dielectric, optical and mechanical properties of perovskites in the form of crystals, ceramics, and thin films are well documented and still studied extensively, the thermodynamic data for condensed and vapour phases that describe the stability of the perovskite systems are scarse. The Knudsen effusion mass spectroscopy and calorimetry used in the thesis allowed to determine the entropy, enthalpy, Gibbs energy, and chemical potential of the condensed phase, as well as the thermo-chemical activity in the vapour phase. Ferroelectric lead titanate PbTiO3 pure and Mn-doped, and the antiferroelectric sodium niobate NaNbO3 pure and also Mn-doped were chosen as materials for investigations.

2. Remarks on PbTiO3

While the thermodynamic data for PbTiO3 can be found in literature and the data by Kobertz are well completed and more accurate, it seems to me that data concerning PbTiO3 doped with Mn are much more important.

First of all tables 4.1 and 6.1 on pages 47 and 90, respectively, are interesting. They contain data obtained from the traces element analysis by ICP-MS (Inductively Coupled Plasma - Mass Spectroscopy) methods. On the one hand, it is not a surprise that single PbTiO3 crystals were contaminated by 15 or even 40 foreign elements, on the other hand there is not - to some extend - sufficient discussion in the text, concerning these results. For example in table 6.1, the concentration of wolfram W is of the order of 1300 ppm-wt, while in the case of others the level of 100. Should we really neglect this W concentration and call W a trace element?

As very interesting one can consider the investigations of concentration of Mn ions introduced on purpose into lead titanate crystal lattice. It is surprising that nominal concentration differs so much from that found by the above mentioned technique. The fact is that it sheds light on the question which ions in the host crystal lattice are replaced by Mn (i.e. Mn can replace both the Ti and Pb ions) but this result warns very strongly about using the term “nominal concentration”. 3. Remarks on NaNbO3 and NaNbO3:Mn

Contents of the chapters on these materials prove that also in this case the goal of the thesis has been achieved, indeed. As previously, the experiments and literature data were thoroughly discussed. It could be said, however, that from the phase transition point of view, no new information was given. Perhaps an important result is that additional phase transition at around 600K was not, as suggested in literature, observed.

In my opinion the fact described by the author in the following manner is interesting : “A temperature-dependent relation of thermodynamic activity of sodium in the NaNbO3 was experimentally obtained for Na by comparing the partial vapour pressures over pure Na2O(c) and NaNbO3 (c), respectively. It can be derived from the pressure data and activity value that NaNbO3 (c) is stable for years at oxygen partial pressure under the equilibrium state, even in the temperature range in the vicinity of the last transition to paraelectric cubic .phase at 907K. Conditions that reduce the partial pressure of oxygen below the equilibrium state (e.g. vacuum or inert gas) diminish the stability. In contrast to this, processes in an atmosphere with higher oxygen partial pressures promote the stability.” In the light of what is cited above one may ask: why, in so many papers on ABO3 perovskites, is it stressed that above 500oC the material is unstable because of the volatility of oxides (e.g. Na2O)? How to control, thus, the stoichiometry in these kinds of materials? Which temperature cannot be overstepped? As in the case of PbTiO3:Mn the results concerning the Mn concentration in the crystal lattice warns about using the term “nominal concentration”. It means that Mn doping in NaNbO3 has nothing to do with initial powders and the final concentration of Mn in crystal volume is by far not perfect, in terms of the desired Mn-concentration. Because it was found that the prepared samples had concentration of Mn ions 10 or even 100 times lower than that initially wanted. 4. The most important results of the thesis:

a) Complete thermodynamic data for PbTiO3, PbTiO3:Mn, NaNbO3 and NaNbO3:Mn single crystals

b) During high temperature heat treatment of the ABO3 sample, the loss of material is stoichiometrically proportional and the chemical composition is not degraded.

c) Oxidic perovskite preparation is at its best, if the oxygen partial pressure of the system is in equilibrium with other reactants.

5. Main question connected with the thesis

On page 122 the author has provided the following sentence (de facto he has repeated this sentence a few times, in different chapters): “The most stable form of a system under constant pressure, a state where nothing more will happen, is called the equilibrium state.” No further details or comments have been given on this sentence. Hence, I would like to know the author’s answer to the following question: what is the equilibrium state for the oxidic ABO3 perovskites and how to achieve/obtain it?

6. Other Remarks

a) It has to be stressed that the thesis by Dietmar Kobertz is a separate part of his broad studies of thermodynamic properties of perovskites and other materials, too. In During his career he has published 21 papers that can be found in the ISI Web of Knowledge database and in the list of journals accepted by the Polish Ministry of Science and Higher Education, which have been cited over 200 times (without self-citations). Also the fact that his h index is equal to 9 means that he publishes essential papers ignoring the ironic rule characterising the contemporary community of scientists, i.e. “publish or perish”.

b) The thesis is well written and can be read with ease by scientists from other fields. As far as I am able to estimate it, the references are those needed and properly cited in the text, I would also expect that the author of the thesis would indicate his share in the papers published.

c) The list of abbreviations does not contain the very important, from the point of view of the thesis goals, abbreviation “ppm-wt”. It is a pity that it has been overlooked, because the thesis is so neatly prepared and so well organised.

d) Does the ferro- or antiferroelectricity order play a role in the thermodynamic description of the phase transitions that these materials undergo?

e) A detailed question: what is the difference between the terms “experimental” and ”empirical” ? (see Table (8.6))

Irrespective of the high value of thesis and conclusions based on it, I would like to know the opinion of the author about universality of the experimental methods he used in his thesis. I am interested in getting an answer because very often the technological methods and quality of materials used in experiments are not well described or even not appreciated enough. 7. Final conclusion

At the end of this review I declare that the thesis fulfils the conditions required by Polish law and a public defence can take a place. Moreover, I would like to make a motion to the Council of the Institute of Physics to distinguish the thesis of Dietmar Kobertz.

prof. Krystian Roleder Department of Ferroelectric Physics Institute of Physics University of Silesia in Katowice