Research

Five lines in thirty-eight years, from adiabatic calorimetry to infrared emissivity. They share a habit: let the data decide the symmetry, and keep the computation reproducible.

My research began in 1988 in the adiabatic calorimetry laboratory of the Faculty of Science, grew into the crystallography of perovskite oxides through the Bilbao Crystallographic Server and the European neutron and synchrotron sources, and since 2018 lives in the Thermomat group, which I co-lead, around the HAIRL emissometer. What follows is the record: the lines, the experiments at large facilities, the projects and groups that paid for them, and the contracts with industry. The publications are on their own page, generated from a registry synchronised with OpenAlex.

Adiabatic calorimetry and structural phase transitions (1988–1995)

My doctoral work, with Ángel López-Echarri and Isabel Ruiz-Larrea, was on the solid–solid phase transitions of ferroic, ferroelastic and incommensurate crystals measured by adiabatic calorimetry: [N(CH₃)₄]₂ZnBr₄ and [N(CH₃)₄]₂ZnI₄, thiourea, betaine calcium chloride dihydrate, N(CH₃)₄CdBr₃, Cs₂ZnI₄ and Cs₂CdBr₄. The instrument was an automatic adiabatic calorimeter working between 20 and 370 K, built and programmed in the laboratory. The method separated the harmonic and anharmonic lattice contributions with Raman, elastic and thermal-expansion data, so that the entropy of each transition could be assigned without ambiguity and the order–disorder or displacive character settled. Stays at the University of Bordeaux with Michel Couzi added the Raman side. The results appeared in physica status solidi (b), Thermochimica Acta, Solid State Communications, Physical Review B, Phase Transitions, Ferroelectrics and Journal of Physics: Condensed Matter, and the thesis was defended in 1994.

Pseudosymmetry and the prediction of phase transitions (1996–2001)

With Mois I. Aroyo and J. Manuel Pérez-Mato I developed a systematic procedure for finding pseudosymmetric structures in crystallographic databases: determine the minimal supergroups of a space group, apply their additional operations to the known structure, and measure how far the transformed structure lies from the original. A small distance reveals a slightly distorted phase of higher symmetry, and therefore a probable phase transition at higher temperature. Applied to the P2₁2₁2₁ and Pnma structures of the Inorganic Crystal Structure Database (Physical Review B 1996, Acta Crystallographica B 1999, Ferroelectrics 1997 and 2000), the method recovered the compounds with known transitions and proposed dozens of new candidates, including displacive ferroelectrics. It became the program PSEUDO on the Bilbao Crystallographic Server, and its logic is how the next line began: the Sr₂MWO₆ family was chosen because pseudosymmetry said it should transform.

Double perovskites and mode crystallography (2003–2019)

From 2003 the line settled on double and triple perovskite oxides, A₂BB′O₆ and their relatives, which distort from the ideal cubic structure through octahedral tilts, cation order and off-centre displacements, and whose magnetic, dielectric and transport properties depend on those distortions: the tungstates Sr₂MWO₆ and Ca₂MWO₆, the antimonates A₂MSbO₆ with M from scandium to the lanthanides, the ruthenates SrLnMRuO₆ and La₂NiRuO₆, the tellurates Sr₂MTeO₆ and their solid solutions, the triple perovskites ALn₂CuTi₂O₉, Na₀.₅K₀.₅NbO₃ and La₂CoMnO₆. For every family the questions were the same: the room-temperature structure and degree of cation order, the sequence of temperature-driven transitions, typically P2₁/n → I4/m → Fm3̄m or through R3̄ and I2/m, and the modes that drive them. The data came from laboratory X-rays, synchrotron radiation at the ESRF, neutrons at the ILL, SINQ, FRM II and the Helmholtz-Zentrum Berlin, high-temperature and high-pressure Raman spectroscopy with Bouchaib Manoun and Peter Lazor, and magnetic measurements.

The method that ties the work together is symmetry-mode analysis, or mode crystallography: describing a distorted structure not by atomic coordinates but by the amplitudes of the symmetry-adapted modes of the parent phase, as implemented in AMPLIMODES on the Bilbao Crystallographic Server. My own contribution, developed on K₀.₅Na₀.₅NbO₃ and the Sr₂MM′O₆ families with M′ = Sb, W, Mo, Ru, Te, is a parametrised refinement protocol: high-resolution data on one member of a family fix the relations among mode amplitudes, and those relations let laboratory X-ray data, which cannot resolve light elements or subtle tilts on their own, be refined to a physically meaningful solution. It reduces the need for expensive high-resolution experiments and makes preliminary data usable. This line produced four of my five doctoral theses (Gateshki 2003, Faik 2009, Iturbe-Zabalo 2012, Orayech 2015), some forty papers, and a lasting collaboration with Morocco, with Abdeslam El Bouari, Bouchaib Manoun and Abdessamad Faik, that continues today with the Sr₂(Co,Fe,Ni)TeO₆ series in Dalton Transactions (2022, 2023).

Thermal radiative properties of materials (2018–)

Over the last decade my activity has moved to the high-temperature thermophysical properties and the infrared radiative behaviour of materials for energy, in Thermomat, the research group on the thermophysical properties of materials at EHU, which I co-lead. The group operates HAIRL, a high-accuracy infrared radiometer built in-house: directional spectral emissivity from 0.83 to 25 μm, from room temperature to 1000 °C, in controlled atmosphere, with a full uncertainty budget. My part of the work is on the instrument and its data as much as on the samples: the upgrade of the emissometer, the model-free cascaded temperature control published in IEEE Transactions on Control Systems Technology, the updated measurement method and uncertainty budget in Metrologia, the “fer” framework for FAIR-compliant reporting of traceable experimental results in IEEE Transactions on Instrumentation and Measurement, and EKHI, the open database of optical and thermal radiative properties of solids described in Scientific Data and indexed by emissivity.org. The materials run from oxidising steels, high-entropy alloys and materials for nuclear fusion to laser-patterned surfaces and selective coatings for concentrated solar power, in close collaboration with CIC Energigune, Tecnalia, Petronor, ArcelorMittal and ESS Bilbao, and with academic partners abroad.

The same years opened a second energy strand with CIC Energigune and Abdessamad Faik: molten nitrate salt nanofluids for thermal energy storage in concentrated solar plants, their enhanced heat capacity and thermal conductivity, the stability of the dispersions and the corrosion of carbon steel in contact with them. It was the fifth doctoral thesis (Nithiyanantham 2019) and five papers in Solar Energy, Solar Energy Materials and Solar Cells, Journal of Energy Storage and Applied Thermal Engineering, with a sixth on thermochemical storage in double hydrated salts.

Computation in physics education

The fifth line began as a way of teaching thermodynamics and became research. Jupyter notebooks written in Basque are the laboratory notebook of a theoretical subject: students derive, compute and plot in the same document, and the subject keeps a daily log of what was done. MinervaLab, an interactive simulation laboratory for first-order phase transitions and the van der Waals fluid, was built as a bachelor’s thesis under my direction, released under the GPL, and is the basis of the physics-education contributions I have presented at CINTE 2016, CSEDU 2017, GIREP 2018 and the 2019 biennial meeting of the Real Sociedad Española de Física, and of the teaching-innovation project on the electronic laboratory notebook I led at EHU. The same tools now answer another question: what the data say about how students perform, from the subject’s own records to the physics entrance examination across the Spanish regions.

Experiments at large European facilities

Nineteen experiments between 2007 and 2011 at the Institut Laue-Langevin in Grenoble (instruments D1B, D2B, D20 and D15), the SINQ spallation source of the Paul Scherrer Institut (HRPT), the FRM II reactor in Garching (SPODI), the Helmholtz-Zentrum Berlin (E9) and the ESRF (BM25 SpLine), sixteen of them as principal investigator. Beam time at these facilities is awarded by international peer review, with success rates near one in two at the ILL and stricter quotas for foreign groups elsewhere; valued at the facilities’ own published cost per day of operation, about 12 000 €, the time awarded amounts to half a million euros. Each proposal was a study of structures and phase transitions in the double-perovskite line, and its data went into the theses and papers above.

Role: PI, principal investigator of the proposal; co-investigator on the three proposals led by Vicente Recarte (Universidad Pública de Navarra).

Competitive projects and research groups

More than forty competitive projects and recognised research groups across the career, and over sixty entries once funded actions and contracts are counted: first as a member of the structural-physics group led by J. Manuel Pérez-Mato and Gotzon Madariaga, since 2018 in Thermomat, and as principal investigator of the EHU node of the ELKARTEK storage project, of the large-facility proposals above and of the teaching-innovation project on the electronic laboratory notebook. The recognised groups are listed first, then the projects in reverse chronological order. Amounts are the sums awarded to EHU as they appear in the certificates of the Vice-Rectorate for Research.

Recognised research groups

Projects and funded actions

Contracts with industry

Research and consultancy contracts of the Thermomat group in which I take part, all on infrared emissivity and thermo-optical measurement.

Community

Around a hundred conference contributions since 1990, with invited talks at the Gordon Conference on order and disorder in solids (New London, 1994), the Kraków workshop on symmetry analysis in diffraction (1996), the Hünfeld symposium on the predictability of crystal structures (1997) and, in the emissivity years, THERMEC, EUROMAT, the MRS Spring Meeting, MMC and EOSAM. Reviewer for some twenty indexed journals in materials science, thermophysics, crystallography and instrumentation, with more than a hundred reviews completed; associate editor of Energies. Member of the organising committee of the two international schools of the Bilbao Crystallographic Server (2009, 2011) and a graduate of the ILL FullProf school, the Hercules course and the European School on Multiferroics. Vice-president of the local section of the Real Sociedad Española de Física.

Conference contributions listed in the dossier and the 2022 CV