Seminario di Kledi Xhaxhiu
Ilaria Fratoddi is pleased to invite you to the seminar:
Syntheses of mixed-valence inorganic/solid state compounds and their structural characterization
Department of Chemistry, University of Tirana, Albania
Topic 1: Syntheses of mixed-valence inorganic/solid state compounds and their structural characterization
The ternary compounds In5Ch5X (Ch = S, Se; X = Cl, Br) are synthesized by solid state methods in evacuated quartz ampoules. They represent new mixed valence solids with indium occurring simultaneously in three different oxidation states: In+ , In3+ and In2+. Despite their similarity in the ionic formulation (In5Ch5X = In+ 2In3+ (In2) 4+ 5Ch2- X - ), which is the same for all compounds, they crystallize in two structure types (In5Ch5Cl-type: monoclinic, P21/m, Z = 2; In5Ch5Br-type: orthorhombic, Pmn21, Z = 2). Disregarding the distribution of the halogen atoms, the anionic partial structure is very similar in both structure types. The main difference is an exchange of (In+ ) by (In2) 4+ and vice versa on selected positions. This is possible due to the similar coordination of both ions (three-capped trigonal prisms). In contrast to the perfectly ordered crystals, HRTEM investigations show remarkable differences in the real structures of In5Ch5Cl and In5Ch5Br, where the chloride containing compounds revealed several structural defects atypical for the In5Ch5Brtype.
Topic 2: Thermoelectric and photoelectric properties of In5Ch5X (Ch = S, Se; X = Cl, Br) and their tuning by targeted substitutions
The mixed valence compounds In5S5Br, In5S3Se2Br, In5SSe4Br and In5Se5Br exhibit interesting thermoelectric properties. A continuous increase of the Seebeck potential is associated with an increase of the selenium content in the structure of In5S5Br. In5S5Br reveals low Seebeck potentials within the studied range (ΔT = 0 - 80K) and a maximum value of 0.34 mV for ΔT = 80K. It behaves as p-type semiconductor. The substitution of two sulphur species by selenium, as in In5S3Se2Br, shows n-type conductivity and approx. -16.00 mV for the same ΔT. Further substitutions of sulphur in the structure maintain the n-type conductivity and increase dramatically the Seebeck potential to -225.26 mV (ΔT = 80K). Repeating cycles of Seebeck potential jump over time for In5Se5Br and In5SSe4Br, show differences in their potential and shape of maxima as well as in their recovery time. I-U measurements on the individual micro needle-shaped crystals, of the ternary mixed valence compound In5Se5Cl, crystallizing in the monoclinic crystal system (P21/m), showed significant light sensitivities. Micro needles of In5Se5Cl “glued” on Si- and Cu- substrates were measured under five different wavelengths with various irradiation intensities to investigate their photo chromatic sensing behavior along with the substrate’s influence. In the measuring voltage range 0 – 3 V and maximal LED illumination intensity, current jumps above two orders of magnitude were observed for white light (4100 K; 200 lm), followed by the blue light (460 nm; 976 mW). The amber light (590 nm; 203 mW) exhibited the lowest response. Green- and blue light were selected to determine the substrate’s effect. The micro-needles chips prepared with Si-substrates displayed higher currents for the same voltages in comparison to those prepared with Cu-substrates. These differences decreased with the voltage increase for both employed wavelengths. The mutual structural substitution of a selenium with sulfur, led to the compound In5SSe4Cl. The later crystallizes similarly to In5Se5Cl. Its I-U measurements recorded with green and blue light, on Cu-substrate chips within the voltage range 0 -3 V, revealed more pronounced photo chromatic sensorial for both wavelengths used. These differences increased with the voltage increase for both employed wavelengths. At 3 V, current increases up to 2.2 times and 2.4 times were observed for the green light and blue light respectively. Typically high and better distinguishable sensorial activity was observed for all the employed wavelengths using chips with In5SSe4Cl crystals on Cusubstrate, even at minimal irradiation LED power (0.25 %).
The seminar will be held in Parravano Room (floor 1st, Building S. Cannizzaro) on Thursday 28 April at 2.30 pm.
It can be followed on Google Meet platform at link meet.google.com/hkc-fhtq-cie.
Kledi Xhaxhiu is Full time lecturer and researcher at the Faculty of Natural Sciences, University of Tirana.
He is specialized in crystallography, vibrational spectroscopy and colloid chemistry.