Appl. Phys. A 74 [Suppl.], S1624–S1626 (2002) / Digital Object Identifier (DOI) 10.1007/s003390101259 Applied Physics A Materials Science & Processing Doping dependence of the tetragonal–orthorhombic phase transition in the superconducting compound La 2-x Sr x CuO 4±δ R. Gilardi 1, ∗ , J. Stahn 1 , F. Altorfer 1 , N. Momono 2 , M. Oda 2 , J. Mesot 1 1 Laboratory for Neutron Scattering, ETHZ & PSI, Villigen, Switzerland 2 Department of Physics, Hokkaido University, Sapporo 060-0810, Japan Received: 18 July 2001/Accepted: 24 October 2001 –  Springer-Verlag 2002 Abstract. The phase transition from tetragonal to ortho- rhombic structure in the high-temperature superconductor La 2-x Sr x CuO 4±δ (La214) has been studied by means of elas- tic neutron scattering. Both underdoped (x = 0.09) and over- doped (x = 0.17) compounds have been investigated. From the temperature dependence of the superstructure peaks, we are able to estimate the critical exponent β, which is found to increase with increasing x . We also observe that the temperature interval over which the critical behaviour appears to be valid is unusually large and strongly doping- dependent. PACS: 74.72.Dn; 64.60.Fr; 61.12.Ex Since the discovery of high-temperature superconductors (HTSCs) [1], much effort has been made in order to under- stand the phase diagram of these new materials. Figure 1 shows the phase diagram of La214. The structure of La214 consists of a stacking of CuO 2 planes separated by La(Sr)O blocks. Starting from an insulating and antiferromagneti- cally ordered state (AF) around zero doping (x = 0), this compound becomes metallic by introducing charge carriers (holes) to the CuO 2 planes. Upon further doping a super- conducting phase (SC) occurs. At first the superconducting transition temperature T c increases with doping (underdoped region), reaching a maximum value (optimally doped re- gion, for La214: x ∼ 0.15, T c ∼ 40 K). Then T c decreases again (overdoped region) and the superconductivity is finally destroyed. In the underdoped state the electronic properties above T c are characterised by two crossover phenomena around T max and T ∗ (T max > T ∗ > T c ). Both crossover phenom- ena are associated with the development of pseudogaps with different characteristic energies, and many other phys- ical properties (magnetic susceptibility, in-plane resistiv- ity, specific heat, ...) exhibit anomalies at these crossover ∗ Corresponding author. (Fax: +41-56/310-2939, E-mail: raffaele.gilardi@psi.ch) 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0 100 200 300 x SC T(K) AF T N T c T* T 0 T max Yoshizaki et al. [2] Ido et al. [3] Ido et al. [3] Braden et al. [4] our results Fig. 1. Phase diagram of La 2-x Sr x CuO 4±δ temperatures [3, 5, 6]. The occurrence of these pseudo- gaps, their origin and the possible connection to high- T c superconductivity are still debated. In addition to this, La214 crystals exhibit a second-order structural phase tran- sition from a high-temperature tetragonal structure (HTT) to a low-temperature orthorhombic structure (LTO), caused by a staggered tilting of the CuO 6 octahedra around the (010) axis (orthorhombic notation), which is related to the softening of the respective phonon modes [7]. The transition temperature T 0 depends strongly on the dopant concentration x (i.e. on the quantity of Sr replacing La atoms): it decreases with increasing Sr content and vanishes near x = 0.2. In this paper we present a neutron-scattering study of the HTT–LTO phase transition in both underdoped (x = 0.09) and overdoped (x = 0.17) La214 crystals. We will then com- pare our results to other studies.